lag - Do leds (num lock, caps lock, scroll lock) on keyboard light up before being handled by computer or after?

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.