Without 3rd party "overclocking", PS/2 is faster. As mentioned above, USB is locked a 125Hz, while WinXP gives you the option of setting PS/2 to 20Hz, 40Hz, 60Hz, 80Hz, 100Hz, or 200Hz. Obviously, a 200Hz sampling rate is faster than a 125Hz sampling rate making PS/2 faster. Whether or not you notice the difference is another story. It should be noted that XP defaults to a 100Hz sampling rate for PS/2 so for users who didn't up the rate for PS/2, it's not surprising they didn't notice a difference between 100Hz and 125Hz.