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.
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:
|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|
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
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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