I've heard PS/2 was faster (because it's a direct ISA/PCI device / hardware interrupt?), whereas USB can introduce more latency because it has no real-time guarantee and it's packet-based.
>There are a number of interesting results, but the point relevant to this question is that there was a fairly significant variance between keyboards, and all the USB keyboards tested had a longer effective scan interval (18.77 ms - 32.75 ms) than the PS/2 keyboards (2.83 ms - 10.88 ms).
https://superuser.com/questions/16893/do-usb-or-ps-2-keyboar...
>There are a number of interesting results, but the point relevant to this question is that there was a fairly significant variance between keyboards, and all the USB keyboards tested had a longer effective scan interval (18.77 ms - 32.75 ms) than the PS/2 keyboards (2.83 ms - 10.88 ms).