Hmmm, hard to say much without a bit more info.
What other games do you play on that machine? And are you able to run them all maxed out without problems?
And what kind of settings are you using for THIS game? Sometimes certain very specific settings can have a very huge effect on things overall. Shadows, for instance, is one of the first things to turn off if you're finding things are slowing down. Shutting off v-sync can help as well. There may be others, I havent seen the settings for that game.