The profiling information in https://developer.blender.org/T72943#848210 shows that viewport redraws take a lot of time between brush samples, making the brush lag in high poly meshes.
By limiting the redraws to 30fps, we are potentially processing more brush samples without getting the threads interrupted by a viewport redraw, increasing performance.
I some test @LazyDodo did, this patch increases the performance from 40 fps up to 100 fps.
{F8286329}