I get the fastest response time using a PS/2 keyboard with native PS/2 on the motherboard. Problem is that my newest PC doesn't have PS/2 anymore.

The differences between all my keyboards are mere milliseconds. Though most of us gamers would know even those teeny input delays are noticable.

PS/2 is based on interrupts and thus doesn't require any further calculation to make out the input. Traditionally, it's wired directly to the south bridge which containts the keyboard matrix. With USB it needs to go through the separate USB controller and the CPU and then calculate what kind of input was made. For this comparison I'm of course talking about native PS/2 and not a PS/2 keyboard through an USB converter (which basically makes it an USB keyboard).

Good thing is that all newer motherboards can initiate USB input devices and hubs right from the BIOS/EFI, which is faster than through a driver from the OS. This should make the delay almost unnoticeable under good circumstances.