So, this problem has sort of resolved itself, at least for a little while. So I uninstalled both the SMP and GPU client and got a program called FAH GPU Tracker v2, handles up to 8 GPUs and the SMP. More for convenience than anything else. At first when I installed it, I had the same slowdown as before, nothing new there. I also tried running with just the SMP on, no slowdown at all, even with the CPU at 100%. But I figured I'd try something out. A few weeks ago, a game (dunno which) kept buggering me about turning down settings because a game was under-performing, so I had it turn off Aero. The lag with the FAH GPU client was still there, and I couldn't be arsed turning Aero back on again. Anyhow, I turned it back on again. Turned on the GPU client just to check, and low and behold, the lag is gone! At least for 15 minutes it has been... This without any other change whatsoever. Anyone got a plausible explanation for this?