Reaction time is a whole-system number
The milliseconds on a browser reaction test are not only “your reflexes.” The display and input path are part of the measurement too.
What happens between green and the click?
The page schedules a visual change, the browser/compositor presents it on a display refresh, you see it and respond, the input device reports the action, the operating system processes it and the browser delivers the click event back to JavaScript. BenchRush measures the interval around that whole visible/browser loop, so several layers contribute.
Refresh rate changes when the stimulus can appear
A higher refresh cadence gives the browser more opportunities to present a new frame, but it does not subtract a fixed number of milliseconds from every result. Frame scheduling, display processing and the exact moment inside a refresh cycle still matter. You can use the browser refresh-rate estimate to document the environment of your runs.
Why BenchRush uses five rounds
One click can be unusually lucky, distracted or mistimed. Multiple randomized rounds reduce the influence of a single outlier and make a personal baseline more useful. False starts are discarded instead of being turned into an artificial fast score.
Fair comparisons are mostly same-device comparisons
If you want to know whether a setting change helped, keep the device, browser, display mode, mouse and test conditions the same and compare repeated averages. Comparing two strangers on different displays and input paths says much less about human reaction alone.
Do not use it as a health assessment
BenchRush reaction timing is an entertainment/browser benchmark. It is not designed or validated for medical, neurological, driving, employment or safety-critical assessment.