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. It reports on both the real-world user experience of a page (Field Data) and provides diagnostic insights to help developers optimize performance (Lab Data).
Field Data vs. Lab Data
PSI provides two types of data about a page's performance to help you optimize it:
- Field Data: Also known as Real User Monitoring (RUM), this data is sourced from the Chrome User Experience Report (CrUX) and represents the actual experience of real users over the last 28 days. Field data is crucial for measuring whether your site meets the Core Web Vitals thresholds.
- Lab Data: This is performance data collected in a controlled, emulated environment using the open-source Lighthouse engine. It runs on a predefined device configuration with specific network and CPU throttling. Lab data is excellent for debugging performance issues during development.
Core Web Vitals (CWV)
Core Web Vitals are a set of real-world user-centered metrics that Google uses to evaluate a page's user experience. PSI reports on the following primary metrics:
- First Contentful Paint (FCP): Measures the time from when the page starts loading to when any part of the page's content is rendered on the screen.
- Largest Contentful Paint (LCP): Measures the time from when the page starts loading to when the largest image or text block is fully rendered. A good LCP is 2.5 seconds or less.
- Total Blocking Time (TBT): Measures the total amount of time between First Contentful Paint (FCP) and Time to Interactive (TTI) where the main thread was blocked by tasks taking longer than 50ms.
- Cumulative Layout Shift (CLS): Measures the visual stability of a page by calculating the sum total of all individual layout shift scores for every unexpected layout shift that occurs during the page lifetime. 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. Because mobile and desktop environments have vastly different capabilities, scores often differ:
- Mobile Strategy: Emulates a mid-tier mobile device (Moto G Power) connected over an emulated 4G network connection. This strategy uses CPU throttling to model real-world mobile processor limitations.
- Desktop Strategy: Emulates a desktop screen using a fast broadband connection with no CPU throttling.