Just a quick note that may or may not be of use to others depending on hardware. Since the 4.0.x builds I've been seeing massive lag and slowdown whenever an area was selected. The key turned out to be the "Animations" option under Settings, which among other things makes the dotted selection line "crawl" around its perimeter. Turning this off has restored full speed to my Paint.NET UI.
Although there are one or two mentions of slowdown on this forum it doesn't seem to be nearly as endemic as I first assumed so this issue is quite possibly limited to certain combinations of hardware and drivers. My rig is a Win7 64bit PC on an Asus Sabertooth Z87 mainboard, 16GB, i7 at 4.3GHz with a 3GB GeForce GTX 780 Ti. Latest drivers for everything at time of posting. YMMV.
