Even though I have the horse power to play the game without fps drops, whenever I turn on DoF (may be other depth effects too, didn't try them) my FPS tanks down and hardware isn't utilized. However I can hold a steady 60 only on 960x540 resolution.
Having a similar problem with DS2: Deathnitive. Regardless of the effects (none enabled) my FPS dips to low 40s in open areas without hardware utilization as well. Without the dll in the folder, the game runs at full speed.