I have noticed recently that if I am running SK in windowed mode, with another application behind it (in particular google chrome), the SK client will at some point begin to lag and drop a lot of frames. At first I thought minimising chrome was fixing my problem, but then I noticed that even dragging the SK client window a couple of pixels in any* direction would also fix the problem.
Apologies if this has been noted before, I hope this suggestion helps others.
Are you using Win7? And what happens if you kill chrome after the game is launched?