So, thanks to the most recent patches in gimp 2.8 and 2.9, the brush outline issues are pretty close to gone.
What that means for you is that drawing with a 256px brush on a 5000×5000 canvas won’t slow your computer to a crawl anymore. Pretty cool if you ask me. Check progress on the bug below.
There are still some issues with lag in general, but it’s much more bearable than before.