I just downloaded the GSoC paint build this AM. It was going along smooth as butter when suddenly a significant performance lag when painting directly on my object. The speed was fine when painting in the UV Editor. Even tho I restarted Blender a number of times I was getting the same performance.
After an hour or so of fiddling with settings, I did a complete machine re-boot. Now it’s fine again.
Just thought I’d share and maybe save somebody an hour of their life.