It is a known issue that heavy use of the adjustment brush will indeed cause slow downs.
If you need to use the brush a lot, you are better off doing the brushing in Photoshop or Photoshop Elements.
This has nothing to do with the number of photos in your Lightroom catalog.
Here are a couple documents increasing performance in Lightroom.
Pete, those steps to improving performance are fine, but they do not address nor solve the actual problem. The actual problem is that as one uses Lightroom (in my case, LR 6 standalone), performance slows down progressively while editing in the Develop module, to the point where the program is unusable. Exiting and restarting the program helps temporarily, but the slowdown soon recurs. I do not have this problem at all with my previous version of LR, and my system far exceeds the requirements for LR6. It seems that this problem is in the same category as the "Server Busy" error -- Adobe either does not know how to fix it (suggesting incompetence) or Adobe does not care enough to fix it.
If you are performing a lot of brushing or spot healing, it is a limitation of the design, as I explained.