Just wanted to share a problem I've had in case others have similar issues. I'm using LR 6 and have for a while had a lot of problems with a laggy adjustment brush. I investigated a little bit today and noticed that even though using GPU acceleration, my CPU still run at 80% while brushing. Tested turning off GPU acc, and CPU went down to 20-30% and the lag disappered. My theory is that the implementation does something stupid like shuffling data like crazy to/from the GPU during brushing, which would make it less effective than regular CPU processing.