Input lag
Input lag is the time from a physical input, such as a click or key press, to the screen showing its result; it adds up across every part of the chain.
Also known as input latency, input delay, end-to-end latency, click to photon
Not measurable from a browser
A web page cannot read this directly. The paragraphs below say what a page can and cannot see, and which tool or reading covers the rest.
Every step adds to it. The mouse or controller waits for its next report and may debounce its switch, the operating system and browser or game handle the event, the game simulates and renders a frame, the frame waits for the display's next refresh, and the display processes the image and its pixels change. Each part can be a few milliseconds or many, and the total is what you feel.
Only some of it can be measured from a web page. A page can measure the refresh interval and a mouse's report interval and estimate what they add; the reaction time test's model takes half to one and a half refresh intervals for the screen, 8.3 to 25 ms at 60 Hz, plus up to one report interval for the mouse. A busy machine adds delay of its own, which the input delay under load guide shows how to see. The display's processing, pixel response and the switch itself need a photodiode and a sensor on the button.
The common reductions follow from the chain: a higher refresh rate, a game or low-latency mode on a TV, a higher polling rate, a cable instead of Bluetooth for audio and controllers, and a machine that is not saturated by background work. Measure before and after each change rather than adding them all at once.
Covered in full
This page defines the term. The pages below explain it, diagnose it and measure it.
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