USB 2.0 has a theoretical minimum latency of 125us. PS/2 has a theoretical minimum latency of 659us, because the maximum serial clock is 16.7kHz, and the smallest scancode is 11 bits. This ignores considerations of matrix scan rate, debouncing, key travel distance, key activation point, the fact that many PS/2 scancodes are bigger than 11 bits, and the fact that USB can transmit more than one key at a time.
rtings.com measures the keyboard I'm using at 0.5ms latency[0]. I'm confident this is faster than any PS/2 keyboard.
Ooh, just learnt that USB 3 fixes this problem! Never mind.