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 two types of data.
In order to show user experience data for a given page, there must be sufficient data for it to be included in the CrUX dataset. A page might not have sufficient data if it has been recently published or has too few samples from real users. When this happens, PSI will fall back to origin-level granularity, which encompasses all user experiences on all pages of the website. Sometimes the origin may also have insufficient data, in which case PSI will be unable to show any real-user experience data.
Assessing quality of experiences
PSI classifies the quality of user experiences into three buckets: Good, Needs Improvement, or Poor. PSI sets the following thresholds in alignment with the Web Vitals initiative:
Good
Needs Improvement
Poor
FCP
[0, 1800ms]
(1800ms, 3000ms]
over 3000ms
FID
[0, 100ms]
(100ms, 300ms]
over 300ms
LCP
[0, 2500ms]
(2500ms, 4000ms]
over 4000ms
CLS
[0, 0.1]
(0.1, 0.25]
over 0.25
INP (experimental)
[0, 200ms]
(200ms, 500ms]
over 500ms
TTFB (experimental)
[0, 800ms]
(800ms, 1800ms]
over 1800ms
Distribution and selected metric values
PSI presents a distribution of these metrics so that developers can understand the range of experiences for that page or origin. This distribution is split into three categories: Good, Needs Improvement, and Poor, which are represented by green, amber, and red bars. For example, seeing 11% within LCP's amber bar indicates that 11% of all observed LCP values fall between 2500ms and 4000ms.
Above the distribution bars, PSI reports the 75th percentile for all metrics. The 75th percentile is selected so that developers can understand the most frustrating user experiences on their site. These field metric values are classified as good/needs improvement/poor by applying the same thresholds shown above.
Core Web Vitals
Core Web Vitals are a common set of performance signals critical to all web experiences. The Core Web Vitals metrics are FID, LCP, and CLS, and they may be aggregated at either the page or origin level. For aggregations with sufficient data in all three metrics, the aggregation passes the Core Web Vitals assessment if the 75th percentiles of all three metrics are Good. Otherwise, the aggregation does not pass the assessment. If the aggregation has insufficient data for FID, then it will pass the assessment if both the 75th percentiles of LCP and CLS are Good. If either LCP or CLS have insufficient data, the page or origin-level aggregation cannot be assessed.
