Note, this requires a small code change to simulate a slow rebuild of the coursecattree cache. As admin, log in to your test site and go to the Site administration / Courses / Manage courses and categories Open a second browser, log in as a second user account with the moodle/core:viewcourselist capability, and go to the course list ( course/index.php ) Make a code change (just for testing) to course/classes/category.php: Search for the comment:  // Re-build the tree. Immediately above this line, add the following line: sleep(10); debugging('Actually building course category tree'); Using the admin browser, click the up or down arrow on a category to change the category order (changing the order will invalidate the cache). Once this as completed, reload the page. Using the other (user) browser, reload the current page. (You need to do this step within ten seconds - ideally a couple of seconds after reloading the admin browser.) EXPECTED: The admin browser takes 10 seconds then displays debugging info about rebuilding the course category tree. EXPECTED: The other browser finishes slightly faster - it should load immediately after the admin browser does. There should be no debugging info. In the admin browser, open a new tab and go to Site administration / Plugins / Caching / Configuration . Find the coursecattree cache area, and click Purge next to it. Wait for this to finish. In the user browser, reload the page. In the admin browser, reload the Manage courses and categories tab. EXPECTED: The user browser takes 10 seconds then displays the page, including debugging info about rebuilding the course cache. EXPECTED: The admin browser finishes immediately after the user browser does, and there should be no debugging info.