What are your system specs?
Make sure you turn off Anti Aliasing Deffered OFF. That is a real killer. You can also try setting the graphics to Auto, I've had really good luck with that and seems to work well for quite a few people. The game has points where it really churns your graphics card and turning back the settings so you have some "reserve power" is always a good idea. People are trying to top out their settings and when the action gets rough it starts bogging down.