Performance
Chrome User Experience Report
Also called CrUX.
Chrome User Experience Report, or CrUX, is Google's public dataset of real-user Web Vitals collected from opted-in Chrome users. It powers the field section of PageSpeed Insights and the Core Web Vitals report in Search Console.
How it is measured
CrUX reports the 75th percentile of each metric over a rolling 28-day window, split by device type and origin or URL. Data appears only when a page or origin has enough traffic, otherwise PageSpeed Insights shows no field data.
You can read it in PageSpeed Insights, the CrUX API, BigQuery, or the CrUX Dashboard. The BigQuery set is monthly, the API is daily, so pick by how quickly you need to see a change.
Worked example
A local news site deploys a lighter hero on the 3rd. Lab LCP drops from 3,100 ms to 2,200 ms the same day. CrUX still shows a p75 LCP of 3,050 ms for the next two weeks, then slides to 2,300 ms once old samples roll out of the 28-day window.
On the origin level, the blog section has LCP 2,100 ms while the shop section sits at 3,600 ms. The combined origin number averages them into a figure that describes neither.
How it differs
CrUX is one public source of field data, limited to Chrome users who opted in and to traffic above a threshold. Field data in general also covers your own real-user monitoring, which can see every browser, every page, and any custom event. CrUX excludes Safari and Firefox.
Common errors
Expecting a deploy to show up the next day. Reading origin-level numbers as if they apply to one slow template. Assuming no data means a good score. Comparing CrUX p75 to a Lighthouse score as if they were the same measurement. Forgetting that mobile and desktop are separate and the weaker one usually fails first.
In practice
Look up your origin and top five URLs in PageSpeed Insights, note the p75 for each Web Vital, and write the date. If the URL has no CrUX data, add your own RUM so you are not flying blind. Wait out the 28 days before declaring a fix a success or failure.