does the fps drop when you turn? or are you just experieincing actual lag without a change in fps?
if its fps... then I have no idea, you'd have better luck posting on the steampowered.com forums under help and tips since its a directly CSS related problem
are you sure your mouse isn't twitching causing a "stutter" because of an inadequate mouse surface? sometimes black is bad for optical mouses and some surfaces can kill the mouse's performance