Cloudflare
If your site uses Cloudflare as reverse-proxy, make sure that Rocket Loader is off
Rocket Loader bundles mootools incorrect and cause unnecessary loop calls
Debugging
Most browsers bundles with a performance debugger, typically accessible via the hotkey F12
With the performance recorder open, start recording, navigate to the unresponsive page, end the recording after ~10 seconds and export it
Join our Discord guild and post in #help-support with the exported performance file attached