I have complained about this design flaw several times to support. I even created a blank tree with 10,000 media items to demonstrate the problem, but I haven't gotten any acknowledgement.
The good news for you is you only have 1,500 items to categorize. FTM is still relatively efficient with those numbers. (If you had 5,000+ then you would be seeing delays of several hours when working with 20+ items.) As you get more categorized, it will speed up. The fewer items you have in the category you're viewing, the faster it works.
I encourage you and everyone else to recreate this problem for yourself and submit feedback to Ancestry support or directly to the developers. This should be something easily fixed, if we can just get someone who knows what they're doing to look at the problem.