Because the game is broken? People running SLI high end cards and 3.5+GHz quads cant play the game at decent framerates. If you pull around 50 FPS consider yourself lucky.
If you mean sluggish in terms of aiming, then there are a few commands to change, like mouse_accel to 0.
If its just gameplay in terms of action than thats just the game. BC2 was worse though.