ActiveAdmin is slow with a large number of admin files in Rails 5.1.4

Question

Issue:

I am using Rails 5.1.4, Ruby 2.5.1, and ActiveAdmin 1.2.1. My application has around 400 ActiveAdmin resource files, which is causing performance issues. The admin panel loads very slowly, taking 20-30 seconds per page.

If I remove half of the admin files, the application runs smoothly.

Example:

Even a simple dashboard page (with nothing inside) takes 20-30 seconds to load:

# app/admin/dashboard.rb
ActiveAdmin.register_page "Dashboard" do
  menu priority: 1, label: proc { I18n.t("active_admin.dashboard") }
end

I noticed that rendering this file takes 10-12 seconds:

/home/ubuntu/.rvm/gems/ruby-2.5.1/gems/activeadmin-1.2.1/app/views/active_admin/page/index.html.arb

After all actions in ApplicationController have been processed and before rendering begins, something takes around 10 seconds to execute.

I tried the following to diagnose and improve the performance:

• Checked the logs, but they don’t show anything useful before the delay.

• Removed half of the admin files, which significantly improves performance.

• Verified that the delay happens before rendering starts, which suggests something is processing before the logs show controller actions.

I expected that ActiveAdmin would handle a large number of admin files efficiently. However, with ~400 admin files, it takes 20-30 seconds per page, even for simple pages like the dashboard. When I reduce the number of admin files, the application runs smoothly.

I'm trying to find out why this slowdown occurs and how I can optimize ActiveAdmin for better performance.