Why the audit is important
Input responsiveness is a key factor in how users perceive the performance of your app. Apps have 100ms to respond to user input. Any longer than that, and the user perceives the app as laggy. See Measure Performance with the RAIL Model for more information.
See the What the audit tests for section of this doc for an explanation of why this audit tests for a target score of 50ms (rather than 100ms, which is what the RAIL model recommends).
How to pass the audit
To make your app respond to user input faster, you need to optimize how your code runs in the browser. Check out the series of techniques outlined in the Rendering Performance docs. These tips range from offloading computation to web workers in order to free up the main thread, to refactoring your CSS selectors to perform less calculations, to using CSS properties that minimize the amount of browser-intensive operations.
One important caveat of this audit is that it's not a complete measurement of input latency. As explained in the What this doc tests for section of this doc, this audit does not measure how long your app truly takes to respond to a user input. In other words, it does not measure that your app's response to the user's input is visually complete.
To measure this manually, make a recording with the Chrome DevTools Timeline. See How to use the Timeline Tool for more help. The basic idea is to start a recording, perform the user input that you want to measure, stop the recording, and then analyze the flame chart to ensure that all stages of the pixel pipeline are complete within 50ms.
How the audit is implemented
This section explains how this audit is implemented, so that you can understand how the audit's score is calculated.
The RAIL performance model recommends that apps respond to user input within 100ms, whereas Lighthouse's target score is 50ms. Why?
There is a 90% probabililty a user would encounter input latency of the amount that Lighthouse reports, or less. 10% of users can expect additional latency.