User blog comment:Rappy 4187/Technical Update: February 5, 2013/@comment-4018164-20130206191154

There is one main issue. If a category is posted multiple times, it can cause the page loading to be really slow. There used to be a glitch that caused pages' categories to be posted several times when saving an edit, and in some cases, the page size was over 20,000 bytes (i remember one that had about 70,000 bytes, but it was probably vandalism).

Anyway, in the old editing mode, you could copy the necessary categories, using Ctrl+A to mark all the gategories in the category box, delete them, and then paste the basic categories.

I think an update that allows switching to a "category-edit" mode may be helpful in some cases, on which it is needed to remove categories.