diff --git a/.dockerignore b/.dockerignore
index 491fc35..996cb5c 100644
--- a/.dockerignore
+++ b/.dockerignore
@@ -1,2 +1,9 @@
 node_modules
 lib
+Dockerfile
+Dockerfile.arm32v7
+Dockerfile.arm64v8
+README.md
+.all-contributorsrc
+env.example
+.github/
diff --git a/README.md b/README.md
index a04cadd..1a010dd 100644
--- a/README.md
+++ b/README.md
@@ -51,7 +51,7 @@ The official docker images are available on [Dockerhub](https://hub.docker.com/r
 ### CLI
 
 ```
-  Mango - Manga Server and Web Reader. Version 0.23.0
+  Mango - Manga Server and Web Reader. Version 0.24.0
 
   Usage:
 
@@ -86,6 +86,10 @@ log_level: info
 upload_path: ~/mango/uploads
 plugin_path: ~/mango/plugins
 download_timeout_seconds: 30
+library_cache_path: ~/mango/library.yml.gz
+cache_enabled: false
+cache_size_mbs: 50
+cache_log_enabled: true
 disable_login: false
 default_username: ""
 auth_proxy_header_name: ""
@@ -97,12 +101,12 @@ mangadex:
   download_queue_db_path: ~/mango/queue.db
   chapter_rename_rule: '[Vol.{volume} ][Ch.{chapter} ]{title|id}'
   manga_rename_rule: '{title}'
-  subscription_update_interval_hours: 24
 ```
 
 - `scan_interval_minutes`, `thumbnail_generation_interval_hours` and `db_optimization_interval_hours` can be any non-negative integer. Setting them to `0` disables the periodic tasks
 - `log_level` can be `debug`, `info`, `warn`, `error`, `fatal` or `off`. Setting it to `off` disables the logging
 - You can disable authentication by setting `disable_login` to true. Note that `default_username` must be set to an existing username for this to work.
+- By setting `cache_enabled` to `true`, you can enable an experimental feature where Mango caches library metadata to improve page load time. You can further fine-tune the feature with `cache_size_mbs` and `cache_log_enabled`.
 
 ### Library Structure
 
diff --git a/shard.yml b/shard.yml
index 1da553d..0054a23 100644
--- a/shard.yml
+++ b/shard.yml
@@ -1,5 +1,5 @@
 name: mango
-version: 0.23.0
+version: 0.24.0
 
 authors:
   - Alex Ling <hkalexling@gmail.com>
diff --git a/spec/util_spec.cr b/spec/util_spec.cr
index 27d97c2..fb4efd7 100644
--- a/spec/util_spec.cr
+++ b/spec/util_spec.cr
@@ -61,3 +61,13 @@ describe "chapter_sort" do
     end.should eq ary
   end
 end
+
+describe "sanitize_filename" do
+  it "returns a random string for empty sanitized string" do
+    sanitize_filename("..").should_not eq sanitize_filename("..")
+  end
+  it "sanitizes correctly" do
+    sanitize_filename("..  \n\v.\rマンゴー/|*()<[1/2] 3.14 hello world ")
+      .should eq "マンゴー_()[1_2] 3.14 hello world"
+  end
+end
diff --git a/src/config.cr b/src/config.cr
index 332a159..aa818c3 100644
--- a/src/config.cr
+++ b/src/config.cr
@@ -11,6 +11,8 @@ class Config
   property session_secret : String = "mango-session-secret"
   property library_path : String = File.expand_path "~/mango/library",
     home: true
+  property library_cache_path = File.expand_path "~/mango/library.yml.gz",
+    home: true
   property db_path : String = File.expand_path "~/mango/mango.db", home: true
   property scan_interval_minutes : Int32 = 5
   property thumbnail_generation_interval_hours : Int32 = 24
@@ -20,6 +22,9 @@ class Config
   property plugin_path : String = File.expand_path "~/mango/plugins",
     home: true
   property download_timeout_seconds : Int32 = 30
+  property cache_enabled = false
+  property cache_size_mbs = 50
+  property cache_log_enabled = true
   property disable_login = false
   property default_username = ""
   property auth_proxy_header_name = ""
diff --git a/src/library/cache.cr b/src/library/cache.cr
new file mode 100644
index 0000000..d0d3f01
--- /dev/null
+++ b/src/library/cache.cr
@@ -0,0 +1,188 @@
+require "digest"
+
+require "./entry"
+require "./types"
+
+# Base class for an entry in the LRU cache.
+# There are two ways to use it:
+#   1. Use it as it is by instantiating with the appropriate `SaveT` and
+#     `ReturnT`. Note that in this case, `SaveT` and `ReturnT` must be the
+#     same type. That is, the input value will be stored as it is without
+#     any transformation.
+#   2. You can also subclass it and provide custom implementations for
+#     `to_save_t` and `to_return_t`. This allows you to transform and store
+#     the input value to a different type. See `SortedEntriesCacheEntry` as
+#     an example.
+private class CacheEntry(SaveT, ReturnT)
+  getter key : String, atime : Time
+
+  @value : SaveT
+
+  def initialize(@key : String, value : ReturnT)
+    @atime = @ctime = Time.utc
+    @value = self.class.to_save_t value
+  end
+
+  def value
+    @atime = Time.utc
+    self.class.to_return_t @value
+  end
+
+  def self.to_save_t(value : ReturnT)
+    value
+  end
+
+  def self.to_return_t(value : SaveT)
+    value
+  end
+
+  def instance_size
+    instance_sizeof(CacheEntry(SaveT, ReturnT)) + # sizeof itself
+      instance_sizeof(String) + @key.bytesize +   # allocated memory for @key
+      @value.instance_size
+  end
+end
+
+class SortedEntriesCacheEntry < CacheEntry(Array(String), Array(Entry))
+  def self.to_save_t(value : Array(Entry))
+    value.map &.id
+  end
+
+  def self.to_return_t(value : Array(String))
+    ids_to_entries value
+  end
+
+  private def self.ids_to_entries(ids : Array(String))
+    e_map = Library.default.deep_entries.to_h { |entry| {entry.id, entry} }
+    entries = [] of Entry
+    begin
+      ids.each do |id|
+        entries << e_map[id]
+      end
+      return entries if ids.size == entries.size
+    rescue
+    end
+  end
+
+  def instance_size
+    instance_sizeof(SortedEntriesCacheEntry) +  # sizeof itself
+      instance_sizeof(String) + @key.bytesize + # allocated memory for @key
+      @value.size * (instance_sizeof(String) + sizeof(String)) +
+      @value.sum(&.bytesize) # elements in Array(String)
+  end
+
+  def self.gen_key(book_id : String, username : String,
+                   entries : Array(Entry), opt : SortOptions?)
+    entries_sig = Digest::SHA1.hexdigest (entries.map &.id).to_s
+    user_context = opt && opt.method == SortMethod::Progress ? username : ""
+    sig = Digest::SHA1.hexdigest (book_id + entries_sig + user_context +
+                                  (opt ? opt.to_tuple.to_s : "nil"))
+    "#{sig}:sorted_entries"
+  end
+end
+
+class String
+  def instance_size
+    instance_sizeof(String) + bytesize
+  end
+end
+
+struct Tuple(*T)
+  def instance_size
+    sizeof(T) + # total size of non-reference types
+      self.sum do |e|
+        next 0 unless e.is_a? Reference
+        if e.responds_to? :instance_size
+          e.instance_size
+        else
+          instance_sizeof(typeof(e))
+        end
+      end
+  end
+end
+
+alias CacheableType = Array(Entry) | String | Tuple(String, Int32)
+alias CacheEntryType = SortedEntriesCacheEntry |
+                       CacheEntry(String, String) |
+                       CacheEntry(Tuple(String, Int32), Tuple(String, Int32))
+
+def generate_cache_entry(key : String, value : CacheableType)
+  if value.is_a? Array(Entry)
+    SortedEntriesCacheEntry.new key, value
+  else
+    CacheEntry(typeof(value), typeof(value)).new key, value
+  end
+end
+
+# LRU Cache
+class LRUCache
+  @@limit : Int128 = Int128.new 0
+  @@should_log = true
+  # key => entry
+  @@cache = {} of String => CacheEntryType
+
+  def self.enabled
+    Config.current.cache_enabled
+  end
+
+  def self.init
+    cache_size = Config.current.cache_size_mbs
+    @@limit = Int128.new cache_size * 1024 * 1024 if enabled
+    @@should_log = Config.current.cache_log_enabled
+  end
+
+  def self.get(key : String)
+    return unless enabled
+    entry = @@cache[key]?
+    if @@should_log
+      Logger.debug "LRUCache #{entry.nil? ? "miss" : "hit"} #{key}"
+    end
+    return entry.value unless entry.nil?
+  end
+
+  def self.set(cache_entry : CacheEntryType)
+    return unless enabled
+    key = cache_entry.key
+    @@cache[key] = cache_entry
+    Logger.debug "LRUCache cached #{key}" if @@should_log
+    remove_least_recent_access
+  end
+
+  def self.invalidate(key : String)
+    return unless enabled
+    @@cache.delete key
+  end
+
+  def self.print
+    return unless @@should_log
+    sum = @@cache.sum { |_, entry| entry.instance_size }
+    Logger.debug "---- LRU Cache ----"
+    Logger.debug "Size: #{sum} Bytes"
+    Logger.debug "List:"
+    @@cache.each do |k, v|
+      Logger.debug "#{k} | #{v.atime} | #{v.instance_size}"
+    end
+    Logger.debug "-------------------"
+  end
+
+  private def self.is_cache_full
+    sum = @@cache.sum { |_, entry| entry.instance_size }
+    sum > @@limit
+  end
+
+  private def self.remove_least_recent_access
+    if @@should_log && is_cache_full
+      Logger.debug "Removing entries from LRUCache"
+    end
+    while is_cache_full && @@cache.size > 0
+      min_tuple = @@cache.min_by { |_, entry| entry.atime }
+      min_key = min_tuple[0]
+      min_entry = min_tuple[1]
+
+      Logger.debug "  \
+        Target: #{min_key}, \
+        Last Access Time: #{min_entry.atime}" if @@should_log
+      invalidate min_key
+    end
+  end
+end
diff --git a/src/library/entry.cr b/src/library/entry.cr
index 92f4def..43fbb23 100644
--- a/src/library/entry.cr
+++ b/src/library/entry.cr
@@ -1,6 +1,9 @@
 require "image_size"
+require "yaml"
 
 class Entry
+  include YAML::Serializable
+
   getter zip_path : String, book : Title, title : String,
     size : String, pages : Int32, id : String, encoded_path : String,
     encoded_title : String, mtime : Time, err_msg : String?
@@ -46,7 +49,7 @@ class Entry
     file.close
   end
 
-  def to_slim_json : String
+  def build_json(*, slim = false)
     JSON.build do |json|
       json.object do
         {% for str in ["zip_path", "title", "size", "id"] %}
@@ -54,23 +57,15 @@ class Entry
       {% end %}
         json.field "title_id", @book.id
         json.field "pages" { json.number @pages }
+        unless slim
+          json.field "display_name", @book.display_name @title
+          json.field "cover_url", cover_url
+          json.field "mtime" { json.number @mtime.to_unix }
+        end
       end
     end
   end
 
-  def to_json(json : JSON::Builder)
-    json.object do
-      {% for str in ["zip_path", "title", "size", "id"] %}
-        json.field {{str}}, @{{str.id}}
-      {% end %}
-      json.field "title_id", @book.id
-      json.field "display_name", @book.display_name @title
-      json.field "cover_url", cover_url
-      json.field "pages" { json.number @pages }
-      json.field "mtime" { json.number @mtime.to_unix }
-    end
-  end
-
   def display_name
     @book.display_name @title
   end
@@ -81,9 +76,17 @@ class Entry
 
   def cover_url
     return "#{Config.current.base_url}img/icon.png" if @err_msg
+
+    unless @book.entry_cover_url_cache
+      TitleInfo.new @book.dir do |info|
+        @book.entry_cover_url_cache = info.entry_cover_url
+      end
+    end
+    entry_cover_url = @book.entry_cover_url_cache
+
     url = "#{Config.current.base_url}api/cover/#{@book.id}/#{@id}"
-    TitleInfo.new @book.dir do |info|
-      info_url = info.entry_cover_url[@title]?
+    if entry_cover_url
+      info_url = entry_cover_url[@title]?
       unless info_url.nil? || info_url.empty?
         url = File.join Config.current.base_url, info_url
       end
@@ -170,6 +173,16 @@ class Entry
   # For backward backward compatibility with v0.1.0, we save entry titles
   #   instead of IDs in info.json
   def save_progress(username, page)
+    LRUCache.invalidate "#{@book.id}:#{username}:progress_sum"
+    @book.parents.each do |parent|
+      LRUCache.invalidate "#{parent.id}:#{username}:progress_sum"
+    end
+    [false, true].each do |ascend|
+      sorted_entries_cache_key = SortedEntriesCacheEntry.gen_key @book.id,
+        username, @book.entries, SortOptions.new(SortMethod::Progress, ascend)
+      LRUCache.invalidate sorted_entries_cache_key
+    end
+
     TitleInfo.new @book.dir do |info|
       if info.progress[username]?.nil?
         info.progress[username] = {@title => page}
diff --git a/src/library/library.cr b/src/library/library.cr
index a5a4a80..93eac0c 100644
--- a/src/library/library.cr
+++ b/src/library/library.cr
@@ -1,12 +1,38 @@
 class Library
+  include YAML::Serializable
+
   getter dir : String, title_ids : Array(String),
     title_hash : Hash(String, Title)
 
   use_default
 
-  def initialize
-    register_mime_types
+  def save_instance
+    path = Config.current.library_cache_path
+    Logger.debug "Caching library to #{path}"
 
+    writer = Compress::Gzip::Writer.new path,
+      Compress::Gzip::BEST_COMPRESSION
+    writer.write self.to_yaml.to_slice
+    writer.close
+  end
+
+  def self.load_instance
+    path = Config.current.library_cache_path
+    return unless File.exists? path
+
+    Logger.debug "Loading cached library from #{path}"
+
+    begin
+      Compress::Gzip::Reader.open path do |content|
+        @@default = Library.from_yaml content
+      end
+      Library.default.register_jobs
+    rescue e
+      Logger.error e
+    end
+  end
+
+  def initialize
     @dir = Config.current.library_path
     # explicitly initialize @titles to bypass the compiler check. it will
     #   be filled with actual Titles in the `scan` call below
@@ -16,6 +42,12 @@ class Library
     @entries_count = 0
     @thumbnails_count = 0
 
+    register_jobs
+  end
+
+  protected def register_jobs
+    register_mime_types
+
     scan_interval = Config.current.scan_interval_minutes
     if scan_interval < 1
       scan
@@ -25,7 +57,7 @@ class Library
           start = Time.local
           scan
           ms = (Time.local - start).total_milliseconds
-          Logger.info "Scanned #{@title_ids.size} titles in #{ms}ms"
+          Logger.debug "Library initialized in #{ms}ms"
           sleep scan_interval.minutes
         end
       end
@@ -51,11 +83,6 @@ class Library
   def sorted_titles(username, opt : SortOptions? = nil)
     if opt.nil?
       opt = SortOptions.from_info_json @dir, username
-    else
-      TitleInfo.new @dir do |info|
-        info.sort_by[username] = opt.to_tuple
-        info.save
-      end
     end
 
     # Helper function from src/util/util.cr
@@ -66,14 +93,18 @@ class Library
     titles + titles.flat_map &.deep_titles
   end
 
-  def to_slim_json : String
+  def deep_entries
+    titles.flat_map &.deep_entries
+  end
+
+  def build_json(*, slim = false, depth = -1)
     JSON.build do |json|
       json.object do
         json.field "dir", @dir
         json.field "titles" do
           json.array do
             self.titles.each do |title|
-              json.raw title.to_slim_json
+              json.raw title.build_json(slim: slim, depth: depth)
             end
           end
         end
@@ -81,15 +112,6 @@ class Library
     end
   end
 
-  def to_json(json : JSON::Builder)
-    json.object do
-      json.field "dir", @dir
-      json.field "titles" do
-        json.raw self.titles.to_json
-      end
-    end
-  end
-
   def get_title(tid)
     @title_hash[tid]?
   end
@@ -99,6 +121,7 @@ class Library
   end
 
   def scan
+    start = Time.local
     unless Dir.exists? @dir
       Logger.info "The library directory #{@dir} does not exist. " \
                   "Attempting to create it"
@@ -107,14 +130,36 @@ class Library
 
     storage = Storage.new auto_close: false
 
+    examine_context : ExamineContext = {
+      cached_contents_signature: {} of String => String,
+      deleted_title_ids:         [] of String,
+      deleted_entry_ids:         [] of String,
+    }
+
+    @title_ids.select! do |title_id|
+      title = @title_hash[title_id]
+      existence = title.examine examine_context
+      unless existence
+        examine_context["deleted_title_ids"].concat [title_id] +
+                                                    title.deep_titles.map &.id
+        examine_context["deleted_entry_ids"].concat title.deep_entries.map &.id
+      end
+      existence
+    end
+    remained_title_dirs = @title_ids.map { |id| title_hash[id].dir }
+    examine_context["deleted_title_ids"].each do |title_id|
+      @title_hash.delete title_id
+    end
+
+    cache = examine_context["cached_contents_signature"]
     (Dir.entries @dir)
       .select { |fn| !fn.starts_with? "." }
       .map { |fn| File.join @dir, fn }
+      .select { |path| !(remained_title_dirs.includes? path) }
       .select { |path| File.directory? path }
-      .map { |path| Title.new path, "" }
+      .map { |path| Title.new path, "", cache }
       .select { |title| !(title.entries.empty? && title.titles.empty?) }
       .sort! { |a, b| a.title <=> b.title }
-      .tap { |_| @title_ids.clear }
       .each do |title|
         @title_hash[title.id] = title
         @title_ids << title.id
@@ -123,8 +168,15 @@ class Library
     storage.bulk_insert_ids
     storage.close
 
-    Logger.debug "Scan completed"
-    Storage.default.mark_unavailable
+    ms = (Time.local - start).total_milliseconds
+    Logger.info "Scanned #{@title_ids.size} titles in #{ms}ms"
+
+    Storage.default.mark_unavailable examine_context["deleted_entry_ids"],
+      examine_context["deleted_title_ids"]
+
+    spawn do
+      save_instance
+    end
   end
 
   def get_continue_reading_entries(username)
diff --git a/src/library/title.cr b/src/library/title.cr
index 61c9813..9b797f4 100644
--- a/src/library/title.cr
+++ b/src/library/title.cr
@@ -1,13 +1,25 @@
+require "digest"
 require "../archive"
 
 class Title
+  include YAML::Serializable
+
   getter dir : String, parent_id : String, title_ids : Array(String),
     entries : Array(Entry), title : String, id : String,
-    encoded_title : String, mtime : Time, signature : UInt64
+    encoded_title : String, mtime : Time, signature : UInt64,
+    entry_cover_url_cache : Hash(String, String)?
+  setter entry_cover_url_cache : Hash(String, String)?
 
+  @[YAML::Field(ignore: true)]
   @entry_display_name_cache : Hash(String, String)?
+  @[YAML::Field(ignore: true)]
+  @entry_cover_url_cache : Hash(String, String)?
+  @[YAML::Field(ignore: true)]
+  @cached_display_name : String?
+  @[YAML::Field(ignore: true)]
+  @cached_cover_url : String?
 
-  def initialize(@dir : String, @parent_id)
+  def initialize(@dir : String, @parent_id, cache = {} of String => String)
     storage = Storage.default
     @signature = Dir.signature dir
     id = storage.get_title_id dir, signature
@@ -20,6 +32,7 @@ class Title
       })
     end
     @id = id
+    @contents_signature = Dir.contents_signature dir, cache
     @title = File.basename dir
     @encoded_title = URI.encode @title
     @title_ids = [] of String
@@ -30,7 +43,7 @@ class Title
       next if fn.starts_with? "."
       path = File.join dir, fn
       if File.directory? path
-        title = Title.new path, @id
+        title = Title.new path, @id, cache
         next if title.entries.size == 0 && title.titles.size == 0
         Library.default.title_hash[title.id] = title
         @title_ids << title.id
@@ -57,24 +70,141 @@ class Title
     end
   end
 
-  def to_slim_json : String
+  # Utility method used in library rescanning.
+  # - When the title does not exist on the file system anymore, return false
+  #     and let it be deleted from the library instance
+  # - When the title exists, but its contents signature is now different from
+  #     the cache, it means some of its content (nested titles or entries)
+  #     has been added, deleted, or renamed. In this case we update its
+  #     contents signature and instance variables
+  # - When the title exists and its contents signature is still the same, we
+  #     return true so it can be reused without rescanning
+  def examine(context : ExamineContext) : Bool
+    return false unless Dir.exists? @dir
+    contents_signature = Dir.contents_signature @dir,
+      context["cached_contents_signature"]
+    return true if @contents_signature == contents_signature
+
+    @contents_signature = contents_signature
+    @signature = Dir.signature @dir
+    storage = Storage.default
+    id = storage.get_title_id dir, signature
+    if id.nil?
+      id = random_str
+      storage.insert_title_id({
+        path:      dir,
+        id:        id,
+        signature: signature.to_s,
+      })
+    end
+    @id = id
+    @mtime = File.info(@dir).modification_time
+
+    previous_titles_size = @title_ids.size
+    @title_ids.select! do |title_id|
+      title = Library.default.get_title! title_id
+      existence = title.examine context
+      unless existence
+        context["deleted_title_ids"].concat [title_id] +
+                                            title.deep_titles.map &.id
+        context["deleted_entry_ids"].concat title.deep_entries.map &.id
+      end
+      existence
+    end
+    remained_title_dirs = @title_ids.map do |title_id|
+      title = Library.default.get_title! title_id
+      title.dir
+    end
+
+    previous_entries_size = @entries.size
+    @entries.select! do |entry|
+      existence = File.exists? entry.zip_path
+      Fiber.yield
+      context["deleted_entry_ids"] << entry.id unless existence
+      existence
+    end
+    remained_entry_zip_paths = @entries.map &.zip_path
+
+    is_titles_added = false
+    is_entries_added = false
+    Dir.entries(dir).each do |fn|
+      next if fn.starts_with? "."
+      path = File.join dir, fn
+      if File.directory? path
+        next if remained_title_dirs.includes? path
+        title = Title.new path, @id, context["cached_contents_signature"]
+        next if title.entries.size == 0 && title.titles.size == 0
+        Library.default.title_hash[title.id] = title
+        @title_ids << title.id
+        is_titles_added = true
+        next
+      end
+      if is_supported_file path
+        next if remained_entry_zip_paths.includes? path
+        entry = Entry.new path, self
+        if entry.pages > 0 || entry.err_msg
+          @entries << entry
+          is_entries_added = true
+        end
+      end
+    end
+
+    mtimes = [@mtime]
+    mtimes += @title_ids.map { |e| Library.default.title_hash[e].mtime }
+    mtimes += @entries.map &.mtime
+    @mtime = mtimes.max
+
+    if is_titles_added || previous_titles_size != @title_ids.size
+      @title_ids.sort! do |a, b|
+        compare_numerically Library.default.title_hash[a].title,
+          Library.default.title_hash[b].title
+      end
+    end
+    if is_entries_added || previous_entries_size != @entries.size
+      sorter = ChapterSorter.new @entries.map &.title
+      @entries.sort! do |a, b|
+        sorter.compare a.title, b.title
+      end
+    end
+
+    true
+  end
+
+  alias SortContext = NamedTuple(username: String, opt: SortOptions)
+
+  def build_json(*, slim = false, depth = -1,
+                 sort_context : SortContext? = nil)
     JSON.build do |json|
       json.object do
         {% for str in ["dir", "title", "id"] %}
         json.field {{str}}, @{{str.id}}
       {% end %}
         json.field "signature" { json.number @signature }
-        json.field "titles" do
-          json.array do
-            self.titles.each do |title|
-              json.raw title.to_slim_json
+        unless slim
+          json.field "display_name", display_name
+          json.field "cover_url", cover_url
+          json.field "mtime" { json.number @mtime.to_unix }
+        end
+        unless depth == 0
+          json.field "titles" do
+            json.array do
+              self.titles.each do |title|
+                json.raw title.build_json(slim: slim,
+                  depth: depth > 0 ? depth - 1 : depth)
+              end
             end
           end
-        end
-        json.field "entries" do
-          json.array do
-            @entries.each do |entry|
-              json.raw entry.to_slim_json
+          json.field "entries" do
+            json.array do
+              _entries = if sort_context
+                           sorted_entries sort_context[:username],
+                             sort_context[:opt]
+                         else
+                           @entries
+                         end
+              _entries.each do |entry|
+                json.raw entry.build_json(slim: slim)
+              end
             end
           end
         end
@@ -92,34 +222,6 @@ class Title
     end
   end
 
-  def to_json(json : JSON::Builder)
-    json.object do
-      {% for str in ["dir", "title", "id"] %}
-        json.field {{str}}, @{{str.id}}
-      {% end %}
-      json.field "signature" { json.number @signature }
-      json.field "display_name", display_name
-      json.field "cover_url", cover_url
-      json.field "mtime" { json.number @mtime.to_unix }
-      json.field "titles" do
-        json.raw self.titles.to_json
-      end
-      json.field "entries" do
-        json.raw @entries.to_json
-      end
-      json.field "parents" do
-        json.array do
-          self.parents.each do |title|
-            json.object do
-              json.field "title", title.title
-              json.field "id", title.id
-            end
-          end
-        end
-      end
-    end
-  end
-
   def titles
     @title_ids.map { |tid| Library.default.get_title! tid }
   end
@@ -177,11 +279,15 @@ class Title
   end
 
   def display_name
+    cached_display_name = @cached_display_name
+    return cached_display_name unless cached_display_name.nil?
+
     dn = @title
     TitleInfo.new @dir do |info|
       info_dn = info.display_name
       dn = info_dn unless info_dn.empty?
     end
+    @cached_display_name = dn
     dn
   end
 
@@ -205,6 +311,7 @@ class Title
   end
 
   def set_display_name(dn)
+    @cached_display_name = dn
     TitleInfo.new @dir do |info|
       info.display_name = dn
       info.save
@@ -214,11 +321,15 @@ class Title
   def set_display_name(entry_name : String, dn)
     TitleInfo.new @dir do |info|
       info.entry_display_name[entry_name] = dn
+      @entry_display_name_cache = info.entry_display_name
       info.save
     end
   end
 
   def cover_url
+    cached_cover_url = @cached_cover_url
+    return cached_cover_url unless cached_cover_url.nil?
+
     url = "#{Config.current.base_url}img/icon.png"
     readable_entries = @entries.select &.err_msg.nil?
     if readable_entries.size > 0
@@ -230,10 +341,12 @@ class Title
         url = File.join Config.current.base_url, info_url
       end
     end
+    @cached_cover_url = url
     url
   end
 
   def set_cover_url(url : String)
+    @cached_cover_url = url
     TitleInfo.new @dir do |info|
       info.cover_url = url
       info.save
@@ -243,6 +356,7 @@ class Title
   def set_cover_url(entry_name : String, url : String)
     TitleInfo.new @dir do |info|
       info.entry_cover_url[entry_name] = url
+      @entry_cover_url_cache = info.entry_cover_url
       info.save
     end
   end
@@ -262,8 +376,15 @@ class Title
   end
 
   def deep_read_page_count(username) : Int32
-    load_progress_for_all_entries(username).sum +
-      titles.flat_map(&.deep_read_page_count username).sum
+    key = "#{@id}:#{username}:progress_sum"
+    sig = Digest::SHA1.hexdigest (entries.map &.id).to_s
+    cached_sum = LRUCache.get key
+    return cached_sum[1] if cached_sum.is_a? Tuple(String, Int32) &&
+                            cached_sum[0] == sig
+    sum = load_progress_for_all_entries(username, nil, true).sum +
+          titles.flat_map(&.deep_read_page_count username).sum
+    LRUCache.set generate_cache_entry key, {sig, sum}
+    sum
   end
 
   def deep_total_page_count : Int32
@@ -317,13 +438,12 @@ class Title
   #   use the default (auto, ascending)
   # When `opt` is not nil, it saves the options to info.json
   def sorted_entries(username, opt : SortOptions? = nil)
+    cache_key = SortedEntriesCacheEntry.gen_key @id, username, @entries, opt
+    cached_entries = LRUCache.get cache_key
+    return cached_entries if cached_entries.is_a? Array(Entry)
+
     if opt.nil?
       opt = SortOptions.from_info_json @dir, username
-    else
-      TitleInfo.new @dir do |info|
-        info.sort_by[username] = opt.to_tuple
-        info.save
-      end
     end
 
     case opt.not_nil!.method
@@ -355,6 +475,7 @@ class Title
 
     ary.reverse! unless opt.not_nil!.ascend
 
+    LRUCache.set generate_cache_entry cache_key, ary
     ary
   end
 
@@ -416,6 +537,17 @@ class Title
   end
 
   def bulk_progress(action, ids : Array(String), username)
+    LRUCache.invalidate "#{@id}:#{username}:progress_sum"
+    parents.each do |parent|
+      LRUCache.invalidate "#{parent.id}:#{username}:progress_sum"
+    end
+    [false, true].each do |ascend|
+      sorted_entries_cache_key =
+        SortedEntriesCacheEntry.gen_key @id, username, @entries,
+          SortOptions.new(SortMethod::Progress, ascend)
+      LRUCache.invalidate sorted_entries_cache_key
+    end
+
     selected_entries = ids
       .map { |id|
         @entries.find &.id.==(id)
diff --git a/src/library/types.cr b/src/library/types.cr
index 4e83135..4c9dc93 100644
--- a/src/library/types.cr
+++ b/src/library/types.cr
@@ -1,4 +1,12 @@
-SUPPORTED_IMG_TYPES = ["image/jpeg", "image/png", "image/webp"]
+SUPPORTED_IMG_TYPES = %w(
+  image/jpeg
+  image/png
+  image/webp
+  image/apng
+  image/avif
+  image/gif
+  image/svg+xml
+)
 
 enum SortMethod
   Auto
@@ -88,6 +96,18 @@ class TitleInfo
   @@mutex_hash = {} of String => Mutex
 
   def self.new(dir, &)
+    key = "#{dir}:info.json"
+    info = LRUCache.get key
+    if info.is_a? String
+      begin
+        instance = TitleInfo.from_json info
+        instance.dir = dir
+        yield instance
+        return
+      rescue
+      end
+    end
+
     if @@mutex_hash[dir]?
       mutex = @@mutex_hash[dir]
     else
@@ -101,6 +121,7 @@ class TitleInfo
         instance = TitleInfo.from_json File.read json_path
       end
       instance.dir = dir
+      LRUCache.set generate_cache_entry key, instance.to_json
       yield instance
     end
   end
@@ -108,5 +129,12 @@ class TitleInfo
   def save
     json_path = File.join @dir, "info.json"
     File.write json_path, self.to_pretty_json
+    key = "#{@dir}:info.json"
+    LRUCache.set generate_cache_entry key, self.to_json
   end
 end
+
+alias ExamineContext = NamedTuple(
+  cached_contents_signature: Hash(String, String),
+  deleted_title_ids: Array(String),
+  deleted_entry_ids: Array(String))
diff --git a/src/mango.cr b/src/mango.cr
index e8d32a3..8716d04 100644
--- a/src/mango.cr
+++ b/src/mango.cr
@@ -7,7 +7,7 @@ require "option_parser"
 require "clim"
 require "tallboy"
 
-MANGO_VERSION = "0.23.0"
+MANGO_VERSION = "0.24.0"
 
 # From http://www.network-science.de/ascii/
 BANNER = %{
@@ -55,8 +55,10 @@ class CLI < Clim
       Config.load(opts.config).set_current
 
       # Initialize main components
+      LRUCache.init
       Storage.default
       Queue.default
+      Library.load_instance
       Library.default
       Plugin::Downloader.default
 
diff --git a/src/plugin/downloader.cr b/src/plugin/downloader.cr
index 054698e..2800232 100644
--- a/src/plugin/downloader.cr
+++ b/src/plugin/downloader.cr
@@ -23,11 +23,6 @@ class Plugin
       job
     end
 
-    private def process_filename(str)
-      return "_" if str == ".."
-      str.gsub "/", "_"
-    end
-
     private def download(job : Queue::Job)
       @downloading = true
       @queue.set_status Queue::JobStatus::Downloading, job
@@ -42,8 +37,8 @@ class Plugin
 
         pages = info["pages"].as_i
 
-        manga_title = process_filename job.manga_title
-        chapter_title = process_filename info["title"].as_s
+        manga_title = sanitize_filename job.manga_title
+        chapter_title = sanitize_filename info["title"].as_s
 
         @queue.set_pages pages, job
         lib_dir = @library_path
@@ -68,7 +63,7 @@ class Plugin
       while page = plugin.next_page
         break unless @queue.exists? job
 
-        fn = process_filename page["filename"].as_s
+        fn = sanitize_filename page["filename"].as_s
         url = page["url"].as_s
         headers = HTTP::Headers.new
 
diff --git a/src/routes/api.cr b/src/routes/api.cr
index b1fb3b3..ed9bb29 100644
--- a/src/routes/api.cr
+++ b/src/routes/api.cr
@@ -133,24 +133,38 @@ struct APIRouter
     end
 
     Koa.describe "Returns the book with title `tid`", <<-MD
-    Supply the `tid` query parameter to strip away "display_name", "cover_url", and "mtime" from the returned object to speed up the loading time
+    - Supply the `slim` query parameter to strip away "display_name", "cover_url", and "mtime" from the returned object to speed up the loading time
+    - Supply the `depth` query parameter to control the depth of nested titles to return.
+      - When `depth` is 1, returns the top-level titles and sub-titles/entries one level in them
+      - When `depth` is 0, returns the top-level titles without their sub-titles/entries
+      - When `depth` is N, returns the top-level titles and sub-titles/entries N levels in them
+      - When `depth` is negative, returns the entire library
     MD
     Koa.path "tid", desc: "Title ID"
     Koa.query "slim"
+    Koa.query "depth"
+    Koa.query "sort", desc: "Sorting option for entries. Can be one of 'auto', 'title', 'progress', 'time_added' and 'time_modified'"
+    Koa.query "ascend", desc: "Sorting direction for entries. Set to 0 for the descending order. Doesn't work without specifying 'sort'"
     Koa.response 200, schema: "title"
     Koa.response 404, "Title not found"
     Koa.tag "library"
     get "/api/book/:tid" do |env|
       begin
+        username = get_username env
+
+        sort_opt = SortOptions.new
+        get_sort_opt
+
         tid = env.params.url["tid"]
         title = Library.default.get_title tid
         raise "Title ID `#{tid}` not found" if title.nil?
 
-        if env.params.query["slim"]?
-          send_json env, title.to_slim_json
-        else
-          send_json env, title.to_json
-        end
+        slim = !env.params.query["slim"]?.nil?
+        depth = env.params.query["depth"]?.try(&.to_i?) || -1
+
+        send_json env, title.build_json(slim: slim, depth: depth,
+          sort_context: {username: username,
+                         opt:      sort_opt})
       rescue e
         Logger.error e
         env.response.status_code = 404
@@ -159,20 +173,25 @@ struct APIRouter
     end
 
     Koa.describe "Returns the entire library with all titles and entries", <<-MD
-    Supply the `tid` query parameter to strip away "display_name", "cover_url", and "mtime" from the returned object to speed up the loading time
+    - Supply the `slim` query parameter to strip away "display_name", "cover_url", and "mtime" from the returned object to speed up the loading time
+    - Supply the `dpeth` query parameter to control the depth of nested titles to return.
+      - When `depth` is 1, returns the requested title and sub-titles/entries one level in it
+      - When `depth` is 0, returns the requested title without its sub-titles/entries
+      - When `depth` is N, returns the requested title and sub-titles/entries N levels in it
+      - When `depth` is negative, returns the requested title and all sub-titles/entries in it
     MD
     Koa.query "slim"
+    Koa.query "depth"
     Koa.response 200, schema: {
       "dir"    => String,
       "titles" => ["title"],
     }
     Koa.tag "library"
     get "/api/library" do |env|
-      if env.params.query["slim"]?
-        send_json env, Library.default.to_slim_json
-      else
-        send_json env, Library.default.to_json
-      end
+      slim = !env.params.query["slim"]?.nil?
+      depth = env.params.query["depth"]?.try(&.to_i?) || -1
+
+      send_json env, Library.default.build_json(slim: slim, depth: depth)
     end
 
     Koa.describe "Triggers a library scan"
diff --git a/src/routes/main.cr b/src/routes/main.cr
index 57917bb..4aa7da6 100644
--- a/src/routes/main.cr
+++ b/src/routes/main.cr
@@ -41,7 +41,7 @@ struct MainRouter
         username = get_username env
 
         sort_opt = SortOptions.from_info_json Library.default.dir, username
-        get_sort_opt
+        get_and_save_sort_opt Library.default.dir
 
         titles = Library.default.sorted_titles username, sort_opt
         percentage = titles.map &.load_percentage username
@@ -59,12 +59,12 @@ struct MainRouter
         username = get_username env
 
         sort_opt = SortOptions.from_info_json title.dir, username
-        get_sort_opt
+        get_and_save_sort_opt title.dir
 
         entries = title.sorted_entries username, sort_opt
-
         percentage = title.load_percentage_for_all_entries username, sort_opt
         title_percentage = title.titles.map &.load_percentage username
+
         layout "title"
       rescue e
         Logger.error e
diff --git a/src/storage.cr b/src/storage.cr
index 39116b9..32f446a 100644
--- a/src/storage.cr
+++ b/src/storage.cr
@@ -428,12 +428,21 @@ class Storage
     end
   end
 
-  def mark_unavailable
+  # Mark titles and entries that no longer exist on the file system as
+  #   unavailable. By supplying `id_candidates` and `titles_candidates`, it
+  #   only checks the existence of the candidate titles/entries to speed up
+  #   the process.
+  def mark_unavailable(ids_candidates : Array(String)?,
+                       titles_candidates : Array(String)?)
     MainFiber.run do
       get_db do |db|
         # Detect dangling entry IDs
         trash_ids = [] of String
-        db.query "select path, id from ids where unavailable = 0" do |rs|
+        query = "select path, id from ids where unavailable = 0"
+        unless ids_candidates.nil?
+          query += " and id in (#{ids_candidates.join "," { |i| "'#{i}'" }})"
+        end
+        db.query query do |rs|
           rs.each do
             path = rs.read String
             fullpath = Path.new(path).expand(Config.current.library_path).to_s
@@ -449,7 +458,11 @@ class Storage
 
         # Detect dangling title IDs
         trash_titles = [] of String
-        db.query "select path, id from titles where unavailable  = 0" do |rs|
+        query = "select path, id from titles where unavailable = 0"
+        unless titles_candidates.nil?
+          query += " and id in (#{titles_candidates.join "," { |i| "'#{i}'" }})"
+        end
+        db.query query do |rs|
           rs.each do
             path = rs.read String
             fullpath = Path.new(path).expand(Config.current.library_path).to_s
diff --git a/src/util/signature.cr b/src/util/signature.cr
index d1a0040..5ca3e14 100644
--- a/src/util/signature.cr
+++ b/src/util/signature.cr
@@ -48,4 +48,32 @@ class Dir
     end
     Digest::CRC32.checksum(signatures.sort.join).to_u64
   end
+
+  # Returns the contents signature of the directory at dirname for checking
+  #   to rescan.
+  # Rescan conditions:
+  #   - When a file added, moved, removed, renamed (including which in nested
+  #       directories)
+  def self.contents_signature(dirname, cache = {} of String => String) : String
+    return cache[dirname] if cache[dirname]?
+    Fiber.yield
+    signatures = [] of String
+    self.open dirname do |dir|
+      dir.entries.sort.each do |fn|
+        next if fn.starts_with? "."
+        path = File.join dirname, fn
+        if File.directory? path
+          signatures << Dir.contents_signature path, cache
+        else
+          # Only add its signature value to `signatures` when it is a
+          #   supported file
+          signatures << fn if is_supported_file fn
+        end
+        Fiber.yield
+      end
+    end
+    hash = Digest::SHA1.hexdigest(signatures.join)
+    cache[dirname] = hash
+    hash
+  end
 end
diff --git a/src/util/util.cr b/src/util/util.cr
index c4e168a..9f5ffee 100644
--- a/src/util/util.cr
+++ b/src/util/util.cr
@@ -35,6 +35,11 @@ def register_mime_types
     # FontAwesome fonts
     ".woff"  => "font/woff",
     ".woff2" => "font/woff2",
+
+    # Supported image formats. JPG, PNG, GIF, WebP, and SVG are already
+    #   defiend by Crystal in `MIME.DEFAULT_TYPES`
+    ".apng" => "image/apng",
+    ".avif" => "image/avif",
   }.each do |k, v|
     MIME.register k, v
   end
@@ -120,3 +125,22 @@ class String
     match / s.size
   end
 end
+
+# Does the followings:
+#   - turns space-like characters into the normal whitespaces ( )
+#   - strips and collapses spaces
+#   - removes ASCII control characters
+#   - replaces slashes (/) with underscores (_)
+#   - removes leading dots (.)
+#   - removes the following special characters: \:*?"<>|
+#
+# If the sanitized string is empty, returns a random string instead.
+def sanitize_filename(str : String) : String
+  sanitized = str
+    .gsub(/\s+/, " ")
+    .strip
+    .gsub(/\//, "_")
+    .gsub(/^[\.\s]+/, "")
+    .gsub(/[\177\000-\031\\:\*\?\"<>\|]/, "")
+  sanitized.size > 0 ? sanitized : random_str
+end
diff --git a/src/util/web.cr b/src/util/web.cr
index 12459e5..5704ea8 100644
--- a/src/util/web.cr
+++ b/src/util/web.cr
@@ -107,6 +107,26 @@ macro get_sort_opt
   end
 end
 
+macro get_and_save_sort_opt(dir)
+  sort_method = env.params.query["sort"]?
+
+  if sort_method
+    is_ascending = true
+
+    ascend = env.params.query["ascend"]?
+    if ascend && ascend.to_i? == 0
+      is_ascending = false
+    end
+
+    sort_opt = SortOptions.new sort_method, is_ascending
+
+    TitleInfo.new {{dir}} do |info|
+      info.sort_by[username] = sort_opt.to_tuple
+      info.save
+    end
+  end
+end
+
 module HTTP
   class Client
     private def self.exec(uri : URI, tls : TLSContext = nil)