Ok, this is a lesser-known trick that is easy to do.
Download something called
NVTweak which is a tray icon application. It can set hidden variables and it is very safe to use.
1. One setting is Image Settings, which is a general graphic quality option, like High/Med/Low. Set it at the main menu screen, but also check to see if you also need to set it on both the DirectX and OpenGL specific areas.
2. There's an Optimizations section with a dropdown called Presets. In here there are four options, from High Performance to Max Quality or something similar. Choose the highest performance one of course, and again check this in both OpenGL and DirectX.
3. Force Vertical Sync to Off, again in both OpenGL and DirectX to be sure.
4. Anti-Aliasing of course can slow things down, and you can force it off with this app. Anisotropic Filtering should not be underestimated as an FPS killer as well, especially when set to 8x or 16x, so set it to Off or 1x or the lowest setting as well.
If you compare identical screenshots you will see a noticable difference, especially with far-off texture detail. But the degradation isn't really that bad, especially since low FPS kills the "experience" way more than this ever will.
I believe this is NV-specific but it might have ATI support, I don't know. I don't care about ATI cards, though I know most of these options are also present on ATI though setting them might require you use the builtin driver dialogs or a different tool.