My PC processor doesn't go above 40%
AMD Athlon 2 quad-core with 8 gigs RAM and Nvidia 480 GTX 1 GB card.
I get an average of 50 fps. Even on fleet during peak hours.
Not sure why your performance sux, but can't help but wonder, what else is running in your background while you play?
I run this game on 4 different PCs, the one I mentioned above being the best, without any lag problems.
Then again, I built them myself, and have no other programs running, besides Windows & Anti-Virus/Spyware, while playing.