It might be vertical sync, which I believe should now be working properly in the latest patch (haven't tried myself just yet). Basically, if vsync is on, it makes for a better image with no screen tearing, but it'll hurt your fps. So if you check in the options and that's ticked on currently, try turning it off and see if it helps. Just a preliminary guess!
-edit-
Oh, just read other threads and maybe it had something to do with the 'white text' thing that is now fixed.