Increasing amounts of lag

I'm using Lightroom 6.0, and it's gotten slower and slower over time. I thought for a while it was just because my computer was elderly, but then I got a new one with massive amounts of memory and that didn't help. I also thought it was because of the size of my catalogue (over thirteen thousand photos), but I made a new catalogue just to hold images I need to resize for work -- which never has more than about half a dozen pieces in it at a time -- and that runs every bit as slow. Does anyone have suggestions for how to speed it up? Photo editing has gotten to be a deeply unenjoyable process because of how glacially the program runs.

(Before anyone says it: I would *very* much prefer not to go to the cloud-based subscription model.)