You have to remember this is platinum games' first pc game. Its only really optimised for Intel and NVIDIA. AMD users have to brute force it by the looks of things.
Had no problem running it at 60fps with everything turned to max with my i5-2400 / 7970. Allthough, the times that the framerate DID dip, it was because of me slashing dudes up ~100times or i had some background task taking up som CPU time.
I can however report that microstuttering was very obvious in this game, as someone mentioned above.