So, GGGP was less wrong than stated, but for the wrong reasons. Huzzah! That is a rather strange defense. "I thought I was supposed to go south, when actually I was supposed to go north. But, I misread the roadsign and got on a road that goes northeast, so I ended up not too far from my destination. See? I'm not a bad navigator!"
Anyway, in the interest of actual analysis:
Let's go at the low end: 10kHz. And let's pick a beefier keycode, say "R CTRL", that has a 2 byte scan code. No, we're not going to benchmark the hilarious pause key. Get real. The vast majority of the scan codes are 1 byte anyway.
The time to send a key down event should be 2 bytes * 11 clocks, or 22 clock periods at 10kHz. That's 2.2 milliseconds. The time to send a key-up event should be 3 bytes * 11 clocks => 3.3 milliseconds.
Both of those seem pretty fast. The OS itself, though, and whatever layers there are between the PS/2 connector and the OS, will add their own latency. But in a race between a PS/2 keyboard and a USB mouse? I think they're going to both be well under 50ms in any case. 50ms is 20Hz, which is "Intellivision games from the late 70s" speed.