Actually, Keystroke Signal Processing can be measured accurately and consistently, but it does require dedicated hardware to do. The KSP is defined as the time from the switch circuit closing, until the signal reaches the PC via the USB port. Don't confuse this with actuation (time to close the switch contact), or polling rate (how often the keyboard reports to your PC via USB).
Logitech used a dedicated test box based on a raspberry Pi. Keyboards tested have a switch hardwired to the test box. The test box closes the circuit and then measures the time until the signal is detected by the Pi test box. There is variance during testing, but large numbers of test responses yield statistically valid results.
The Logitech G Pro gaming keyboard KSP is 2.5ms on make and on break (time from contact opening to signal received. G810 is close to that speed, but we focused on Pro for the speed optimization.
The Pro polls at 1ms, so the fastest any KSP can be is 1ms. 1ms is the fasted polling rate for PCs running Windows.