The reaction test measures you; this one measures the machine. Every key press or click is timestamped by the operating system — here you can see how long it takes from that timestamp to the page receiving the event, and to the next rendered frame. High or spiky numbers mean a loaded system is sitting between your hand and the screen.
A web page sees your input at the moment the OS hands it over with a timestamp. From there, to event is the queueing and dispatch delay inside the OS and browser, and to frame adds the wait for the next render tick — on a 60 Hz refresh that alone averages ~8 ms, which is why gaming at 120 Hz+ feels snappier even at the same FPS.
What no page can see: the keyboard's matrix scan and debounce (1–10 ms), USB/Bluetooth polling (0.1–8 ms), and your display's processing and pixel response (1–20 ms). The true button-to-photon latency is this measurement plus those stages — so treat the numbers here as the software share of your input lag, the part that changes with system load, browser, and refresh rate.
No — this measures OS timestamp to rendered frame. It can't see keyboard scan time, USB polling, or pixel response; button-to-photon is this plus those hardware stages.
On a healthy machine at 60 Hz, most presses land within a frame or two: roughly 5–25 ms. Consistently higher, or big spikes, usually means a loaded CPU or a power-saving browser mode.
Reaction time measures you — typically 200–300 ms. This measures the machine's share, a small fraction of that.