Hi
I think I know what your problem may be as I had this myself and it totally stumped me…. Do you have your after effects pumping out to an external monitor? Like a broadcast monitor? I know it sounds weird, but when I had my settings to preview out to my broadcast monitor, it was like it was sucking all of the graphics card memory form the computer, even drawing and moving a simple mask was really laggy, once i disabled the setting back to monitor only, everything sped up dramatically. I would try that…..