What is PSI aka Page Speed Insight?

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About PageSpeed Insights
PageSpeed Insights (PSI) reports on the performance of a page on both mobile and desktop devices, and provides suggestions on how that page may be improved.

PSI provides both lab and field data about a page. Lab data is useful for debugging performance issues, as it is collected in a controlled environment. However, it may not capture real-world bottlenecks. Field data is useful for capturing true, real-world user experience – but has a more limited set of metrics. See How To Think About Speed Tools for more information on the 2 types of data.

Performance score
At the top of the report, PSI provides a score which summarizes the page’s performance. This score is determined by running Lighthouse to collect and analyze lab data about the page. A score of 90 or above is considered fast, and 50 to 90 is considered average. Below 50 is considered to be slow.

Real-World Field Data
When PSI is given a URL, it will look it up in the Chrome User Experience Report (CrUX) dataset. If available, PSI reports the First Contentful Paint (FCP) and the First Input Delay (FID) metric data for the origin and potentially the specific page URL.

Classifying Fast, Average, Slow
PSI also classifies field data into 3 buckets, describing experiences deemed fast, average, or slow. PSI sets the following thresholds for fast / average / slow, based on our analysis of the CrUX dataset: