PERF: improve perf of initial payload

also reduce  querying in topic query

app/models/site.rb

@@ -41,7 +41,7 @@ class Site
     @categories ||= begin
       categories = Category
         .secured(@guardian)
-        .includes(:topic_only_relative_url, :subcategories)
+        .includes(:topic_only_relative_url)
         .order(:position)
 
       unless SiteSetting.allow_uncategorized_topics
@@ -50,6 +50,13 @@ class Site
 
       categories = categories.to_a
 
+      with_children = Set.new
+      categories.each do |c|
+        if c.parent_category_id
+          with_children << c.parent_category_id
+        end
+      end
+
       allowed_topic_create = Set.new(Category.topic_create_allowed(@guardian).pluck(:id))
 
       by_id = {}
@@ -62,7 +69,7 @@ class Site
       categories.each do |category|
         category.notification_level = category_user[category.id]
         category.permission = CategoryGroup.permission_types[:full] if allowed_topic_create.include?(category.id)
-        category.has_children = category.subcategories.present?
+        category.has_children = with_children.include?(category.id)
         by_id[category.id] = category
       end
 

lib/guardian/category_guardian.rb

@@ -54,8 +54,11 @@ module CategoryGuardian
 
   # all allowed category ids
   def allowed_category_ids
-    unrestricted = Category.where(read_restricted: false).pluck(:id)
-    unrestricted.concat(secure_category_ids)
+    @allowed_category_ids ||=
+      begin
+        unrestricted = Category.where(read_restricted: false).pluck(:id)
+        unrestricted.concat(secure_category_ids)
+      end
   end
 
   def topic_create_allowed_category_ids

lib/topic_query.rb

@@ -46,6 +46,7 @@ class TopicQuery
     options.assert_valid_keys(VALID_OPTIONS)
     @options = options.dup
     @user = user
+    @guardian = Guardian.new(@user)
   end
 
   def joined_topic_user(list=nil)
@@ -359,7 +360,7 @@ class TopicQuery
         when 'unlisted'
           result = result.where('NOT topics.visible')
         when 'deleted'
-          guardian = Guardian.new(@user)
+          guardian = @guardian
           if guardian.is_staff?
             result = result.where('topics.deleted_at IS NOT NULL')
             require_deleted_clause = false
@@ -391,7 +392,7 @@ class TopicQuery
       result = result.where('topics.posts_count <= ?', options[:max_posts]) if options[:max_posts].present?
       result = result.where('topics.posts_count >= ?', options[:min_posts]) if options[:min_posts].present?
 
-      Guardian.new(@user).filter_allowed_categories(result)
+      @guardian.filter_allowed_categories(result)
     end
 
     def remove_muted_categories(list, user, opts=nil)