Why can a system 3x more powerful not get 3x the performance?
Eh, short answer... 3x the crappy programming! : P
I'm not letting Gearbox off the hook completely, but that's pretty naive. If you played Halo at 640x480 without the added effects present on the PC version and were happy with an average of 30 FPS, you'd get the performance you got from an X Box.