Under Moodle 3.0.7
While helping a teacher with drag and drop issues in her quiz edit page, I realized that the page took a vers long time to load (over a minute).
She was selecting questions form a category that has over 1000 questions. If we display all the questions in the category view (no pagination), it causes the questionbank.ajax.php to load all of the questions. Going back to 20 questions per page fixes it.
I know nothing about Ajax, but it seems strange to me that the way you display your questions in a course category has such an effect on the loading time.
I reproduced the problem on a smaller scale with 100 questions in a category. If all displayed, it takes over 2 seconds to load, while paginated (20 questions per page), the average loading time is around 450 ms.
François