I have to say though, I'm not entirely sure how I would go around measuring the input from a keyboard (such that the keyboard controller is included in the end to end measurement), would key travel time be included etc and how would such device be implemented with cheap availability (so wide number of configurations would be submitted into a results database) and with no need to disassemble existing keyboards or mouses etc.

I think the solution could be a latency calibrated touch sensor pad and audio sensor, so you calibrate the audio input latency using the sensor pad, then subtract the travel time by analyzing the audio but this seems overly complicated.