DEV: Limit preloaded categories (#23958)

Site data is preloaded on the first page load, which includes categories
data. For sites with many categories, site data takes a long time to
serialize and to transfer.

In the future, preloaded category data will be completely removed.

app/models/site.rb

@@ -4,6 +4,9 @@
 class Site
   include ActiveModel::Serialization
 
+  # Number of categories preloaded when lazy_load_categories is enabled
+  LAZY_LOAD_CATEGORIES_LIMIT = 50
+
   cattr_accessor :preloaded_category_custom_fields
 
   def self.reset_preloaded_category_custom_fields
@@ -116,6 +119,10 @@ class Site
              )
             categories << category
           end
+
+          if SiteSetting.lazy_load_categories && categories.size >= Site::LAZY_LOAD_CATEGORIES_LIMIT
+            break
+          end
         end
 
         with_children = Set.new

spec/models/site_spec.rb

@@ -183,6 +183,17 @@ RSpec.describe Site do
         DiscoursePluginRegistry.clear_modifiers!
       end
     end
+
+    context "when lazy_load_categories" do
+      before { SiteSetting.lazy_load_categories = true }
+
+      it "limits the number of categories" do
+        stub_const(Site, "LAZY_LOAD_CATEGORIES_LIMIT", 1) do
+          categories = Site.new(Guardian.new).categories
+          expect(categories.size).to eq(1)
+        end
+      end
+    end
   end
 
   it "omits groups user can not see" do