Keep task manager open while you play the game. You can switch to it and see what's causing the problem - is your CPU at 100 and your GPU at 0%? Is your GPU at 100% and your CPU at 100%? Is your hard drive at 100%? Are your CPU and GPU both at 50%? Your GPU should be at 100% and your CPU should be below 100%.

Report back and let us know where the problem is located, and we'll go from there.