Measure your reaction time in milliseconds: click as soon as the screen turns from red to green, over 5 rounds.
When the box turns green, click as quickly as you can. Your time is measured in milliseconds and averaged over 5 rounds. Clicking before green counts as too soon.
Typical human reaction times to a visual signal are around 250–300 ms. Sleep, caffeine, practice and your screen and mouse all make a difference.
Under 250 ms is fast and under 200 ms is exceptional. Most people average 250–300 ms.
Screen refresh rate and input lag add delay. A 60 Hz screen can add up to about 16 ms.