On the Moodle demo site (https://qa.moodledemo.net/question/bank/managecategories/category.php?cmid=1227) it seems fast. So, what is it about your site that makes it slow?
If you can reproduce why it is slow, then that is probably a bug, and should be reported to the bug tracker.
If you can reproduce why it is slow, then that is probably a bug, and should be reported to the bug tracker.