What is PageSpeed Insights?
PageSpeed Insights (PSI) is a free tool provided by Google that analyzes the content of a web page, then generates suggestions to make that page faster.
Field Data vs. Lab Data
PSI provides two types of data about a page's performance to help you optimize it:
- Field Data: Real user monitoring data from the Chrome User Experience Report (CrUX) collected over 28 days.
- Lab Data: Performance data collected in a controlled environment using the Lighthouse engine.
Core Web Vitals (CWV)
User-centered metrics that Google uses to evaluate a page's user experience:
- First Contentful Paint (FCP): Measures the time until any part of the page's content is rendered on screen.
- Largest Contentful Paint (LCP): Measures when the largest image or text block is fully rendered. A good LCP is 2.5 seconds or less.
- Total Blocking Time (TBT): Measures total time the main thread was blocked by tasks longer than 50ms.
- Cumulative Layout Shift (CLS): Measures visual stability. A good CLS score is 0.1 or less.
Device Strategies: Mobile vs. Desktop
PageSpeed Insights allows you to run reports using either a Mobile or a Desktop strategy:
- Mobile Strategy: Emulates a mid-tier mobile device on an emulated 4G network connection.
- Desktop Strategy: Emulates a desktop screen using a fast broadband connection.