I can't offer an explanation, but a possibly off-the-wall suggestion: check that Auto-mask is not turned on in your brush settings. I've noticed this particular function "struggling" a bit in the past (when using a former underpowered PC) and while your PC's spec seems fine to me, it can do no harm to make the experiment.
If the (non-auto-masked) brush does stray where it shouldn't go, you can always hold Alt key to switch into Erase mode, and get rid of that.
My experience is that LR just runs more fluidly, and the results are smoother, doing it this way.
