The responsiveness of the paint mode depends not only of the point cloud, but also from other elements as the complexity of the area, the surface, etc.
Do you have the same problem creating a new Paint area ? If you edit a very complex region, it may be a bit slow because the perimeter must be evaluated periodically. In this case, may be useful to convert it to spline and create a new area.
If you send us a test scene, we would check if there is some problem.