What's the User Timing API?
Making your web app fast and responsive is crucial for a good user experience. The first step in improving performance is identifying where time is being spent.
Check out the HTML5 Rocks page about the User Timing API for a quick introduction to using it.
How Lighthouse reports User Timing data
When your app uses the User Timing API to add marks (that is, time stamps) and measures (that is, measurements of the elapsed time between marks), you'll see them in your Lighthouse report:
Lighthouse extracts User Timing data from Chrome's Trace Event Profiling Tool.
This audit is not structured as a pass or fail test. It's just an opportunity to discover a useful API that can help you measure your app's performance.
See the Lighthouse performance scoring post to learn how your page's overall performance score is calculated.