Turn off the following. I won't tell you how, because every computer is different.
Ambient Occlusion
Anti-aliasing
Vertical Sync
Shadows
And if you know how to enable triple buffering on your graphics card, do that too.
I'd also recommend, if you're playing on a 1080p monitor, turn your resolution down to 720p.
And finally, this goes without saying, but make sure your graphics card is using its latest drivers.
