I'm trying to find a simple method to measure input lag in games and came up with idea of binding some action (shooting from a gun for example) to caps lock and measuring difference between caps lock led light up and action on a display, but I'm not sure if leds on keyboard light up before being handled by interrupt controller/cpu etc. or after. I tried to measure this on windows logon screen while entering password (windows warns that caps lock is enabled when pressing caps lock key) and it seems that there is about 50-70 ms of delay before led light up and caps lock warning. But at the same time I remember that if a computer hard freezes, leds on keyboard will not react to caps lock/num lock presses.
As far as I know, the LEDs are actually controlled by the operating system. Any lag you measure is actually the lag via the OS. Even when in BIOS screens or while the machine is booting, the keyboard is under the control of the OS.
If you can in fact peg your CPU, you will find that you can in fact slow the time to which the machine responds to CAPS and NUM lock.