1. 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.
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 good. 50 to 90 is a score that needs improvement, and below 50 is considered poor.
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),
- First Input Delay (FID),
- Largest Contentful Paint (LCP), and
- Cumulative Layout Shift (CLS) metric data for the origin and potentially the specific page URL.
Classifying Good, Needs Improvement, Poor
PSI also classifies field data into 3 buckets, describing experiences deemed good, needs improvement, or poor. PSI sets the following thresholds for good / needs improvement / poor, based on our analysis of the CrUX dataset:
| Good | Needs Improvement | Poor | |
|---|---|---|---|
| FCP | [0, 1000ms] | (1000ms, 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 |
Distribution and selected metric values
PSI presents a distribution of these metrics so that developers can understand the range of FCP, FID, LCP, and CLS values for that page or origin. This distribution is also split into three categories: Good, Needs Improvement, and Poor, denoted with green, orange, and red bars.
For example, seeing 14% within FCP's orange bar indicates that 14% of all observed FCP values fall between 1000ms and 3000ms. This data represents an aggregate view of all page loads over the previous 30 days.
Core Web Vitals
Core Web Vitals are a common set of signals critical to all web experiences. The Core Web Vitals metrics are FID, LCP, and CLS, with their respective thresholds. A page passes the Core Web Vitals assessment if the 75th percentiles of all three metrics are Good. Otherwise, the page does not pass the assessment.
Lab data
PSI uses lighthouse to analyze the given URL, generating a performance score that estimates the page's performance on different metrics, including: First Contentful Paint, Largest Contentful Paint, Speed Index, Cumulative Layout Shift, Time to Interactive, and Total Blocking Time.
Each metric is scored and labeled with a icon:
- Good is indicated with a green check mark
- Needs Improvement is indicated with orange informational circle
- Poor is indicated with a red warning triangle
Audits
Lighthouse separates its audits into three sections:
- Opportunities provide suggestions how to improve the page’s performance metrics. Each suggestion in this section estimates how much faster the page will load if the improvement is implemented.
- Diagnostics provide additional information about how a page adheres to best practices for web development.
- Passed Audits indicates the audits that have been passed by the page.
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