Thanks I've used the purge feature. It's those pesky keywords used but a once or twice I'd like to delete.
My suspicion reflects the gradual degradation of performance over the last couple versions. For example, whenever the RAM use gets above 3 or 4 GB (I have 32 GBs of RAM on my i7 Windows 10 PC) LR hangs while it appears to gather information when I Ctrl click on a series of photos that I want to highlight then delete.
Even if it doesn't improve LR's efficiency I suspect it would improve mine should there be a way to cull the heard.
While I agree that culling your keywords is probably a good idea, I still don't see a link with the number of keywords. Its really not obvious how the number of keywords would affect most aspects on Lightroom Classic performance. Nevertheless, if you cull the keywords, let us know if performance improves. (Or try an experiment to see if that helps)