Same problem here. I get about 30-50 fps in single-player and 15-25 fps in multiplayer, regardless of what video settings I choose. Both CPU and GPU usage seem to hover at around 50% and never seem to exceed 60%.

Rig:
i5 2500k
GTX 670
16GB Ram

A couple of additional things I've noticed:

1. I've played with some friends running the game on less powerful rigs (AMD Phenom II X4 955 BE, GTX 460, 4GB Ram, etc...), and they run the game at the same or better fps than me, with higher settings as well...

2. I've noticed that the higher I turn my settings up, the less of my system resources the game uses. Maxed out (everything ultra, max view, shadow, and object draw distance), the game only uses about 20% of my GPU and gives me 15-35 fps.