There was a study in 2002 that evaluated the response times of various keyboards so that those delays could be better accounted for in experiments where subjects' response times were being measured with keyboards.
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).
To explain it simply, keyboards scan across each column of keys and check to see whether any are pressed. So your signal isn't generated the instant you press the key, but rather when the controller scans the key and sees that it is pressed. After the keyboard sends the PC the signal there are obviously additional delays before the character appears on your screen, but those are fixed regardless of the keyboard type.
So if you pressed a key the moment after it was scanned, it could take almost 30ms longer on a slow USB keyboard to be detected and sent to the computer. I'm sure there are some serious gamers who would claim to notice that kind of delay.