Switch completely to Gio for icons and it fixed the startup/lookup time for icons. Now works with every icon theme. Taglist/Tasklist/Window switcher still rely on WM_CLASS and WM_NAME to match the icon name, this probably can't be fixed in a sane way

awesome/src/tools/gio_icon_lookup.lua

@@ -0,0 +1,83 @@
+-- Libraries
+local lgi = require("lgi")
+local Gtk = lgi.require("Gtk", "3.0")
+local Gio = lgi.Gio
+local gears = require("gears")
+local GLib = require("lgi").GLib
+
+-- Get all .desktop files as gobjects
+local app_info = Gio.AppInfo
+local app_list = app_info.get_all()
+
+-- Init a new Gtk theme from the users string
+local gtk_theme = Gtk.IconTheme.new()
+Gtk.IconTheme.set_custom_theme(gtk_theme, User_config.icon_theme)
+
+---Gets the icon path from an AppInfo gicon.
+---@param app Gio.AppInfo
+---@return string path
+function Get_gicon_path(app)
+  local icon_info = gtk_theme:lookup_by_gicon(app, 64, 0)
+  if icon_info then
+    local path = icon_info:get_filename()
+    if path then
+      return path
+    end
+  end
+  return ""
+end
+
+---Takes a class and name string and tries to match it to an icon.
+---@param class string
+---@param name string
+---@return string | nil icon_path
+function Get_icon(class, name)
+  class = string.lower(class) or ""
+  name = string.lower(name) or ""
+  for _, app in ipairs(app_list) do
+    local desktop_app_info = Gio.DesktopAppInfo.new(app_info.get_id(app))
+    local icon_string = Gio.DesktopAppInfo.get_string(desktop_app_info, "Icon")
+    if icon_string then
+      icon_string = string.lower(icon_string)
+      if icon_string:match(class) or class:match(icon_string) then
+        return Get_gicon_path(app_info.get_icon(app))
+      elseif icon_string:match(name) or name:match(icon_string) then
+        return Get_gicon_path(app_info.get_icon(app))
+      end
+    end
+  end
+  return nil
+end
+
+---Will return every $XDG_DATA_DIRS
+---@return table
+local function get_paths()
+  local dirs = {}
+
+  local dir
+  for _, value in ipairs(GLib.get_system_data_dirs()) do
+    dir = GLib.build_filenamev({ value, "applications" })
+    if gears.filesystem.dir_readable(dir) then table.insert(dirs, dir) end
+  end
+
+  dir = GLib.build_filenamev({ GLib.get_user_data_dir(), "applications" })
+  if gears.filesystem.dir_readable(dir) then table.insert(dirs, dir) end
+
+  return dirs
+end
+
+---Returns every .desktop file into a table
+---@param file string .desktop files
+---@return string | nil path
+function Get_desktop_values(file)
+
+  if not file or file == "" then
+    return
+  end
+
+  for _, dir in ipairs(get_paths()) do
+    if gears.filesystem.file_readable(dir .. "/" .. file, "r") then
+      return dir .. "/" .. file
+    end
+  end
+end