Google PageSpeed Insights (PSI) is a free tool that analyzes the performance of a web page on both mobile and desktop devices. It provides a score that summarizes performance and offers specific recommendations to fix issues that slow down the application.
This guide explains how to run a test, interpret the data sources, and understand the metrics provided in the report.
Running a Speed Test
Using PageSpeed Insights is straightforward and requires no installation.
1. Visit the Website: Go to the official PageSpeed Insights page at pagespeed.web.dev.
2. Input URL: Enter the full web address (URL) of the page you wish to test.
3. Analyze: Click the Analyze button. The tool will take a few seconds to collect data and generate the report.
Once the report loads, you can toggle between Mobile and Desktop tabs at the top of the screen to see how the page performs on different devices.
Mobile performance scores are typically lower than desktop scores because mobile devices often have slower processors and rely on less stable network connections.
Understanding Data Types: Field vs. Lab
PSI provides two distinct types of data. Understanding the difference is critical for accurate analysis.
1. Field Data (Real User Experience) This section, labeled "Discover what your real users are experiencing," is based on the Chrome User Experience Report (CrUX). It aggregates anonymized data from real Chrome users who have visited your site over the previous 28 days. This data reflects real-world conditions, including various devices and network speeds.
If a website does not have enough traffic, this section may show "No data available." In such cases, you must rely on Lab Data.
2. Lab Data (Diagnose Performance Issues) This section is generated by Google Lighthouse. It simulates a user visiting the page in a controlled environment with specific network and hardware settings. This data is useful for debugging because it provides detailed diagnostic information and is available immediately, even for sites with zero traffic.
Lab data uses a simulated slow network connection (throttling). Therefore, the "Performance Score" in the lab data might be worse than what your actual users experience on faster networks. Always prioritize Field Data (Core Web Vitals) if it is available.
Interpreting Core Web Vitals
At the top of the report, you will see a Pass/Fail assessment for Core Web Vitals. These are the specific metrics Google uses to evaluate user experience and ranking.
• LCP (Largest Contentful Paint): Measures loading speed. It marks the time when the largest text block or image is rendered.
◦ Good: 2.5 seconds or less.
• INP (Interaction to Next Paint): Measures responsiveness. It checks how quickly the page responds when a user clicks or types.
◦ Good: 200 milliseconds or less.
• CLS (Cumulative Layout Shift): Measures visual stability. It tracks if elements shift around unexpectedly while the page loads (e.g., ads popping in and pushing text down).
◦ Good: 0.1 or less.
The Performance Score
The big number at the top of the report (0–100) is the Performance Score. This is a weighted average of the Lab Data metrics.
• 90–100 (Green): Good.
• 50–89 (Orange): Needs Improvement.
• 0–49 (Red): Poor.
It is very difficult to achieve a 100/100 score consistently. It is better to focus on improving the Core Web Vitals (Field Data) rather than obsessing over a perfect Lab score.
Opportunities and Diagnostics
The bottom section of the report lists Opportunities. These are specific suggestions to speed up the page, such as:
• Serve images in next-gen formats: Using WebP or AVIF instead of heavy JPEGs or PNGs.
• Eliminate render-blocking resources: Deferring JavaScript or CSS that prevents the page from displaying content immediately.
• Reduce unused JavaScript: Removing code that is not needed for the page to function.
Each opportunity includes an estimated time saving if the fix is implemented.
Important Terms Explained
TTFB (Time to First Byte) The time it takes for the browser to receive the very first byte of data from the web server. A high TTFB indicates a slow server connection.
FCP (First Contentful Paint) The time it takes for the first bit of content (text or image) to appear on the screen, letting the user know the page is loading.
Lighthouse An open-source automated tool that runs the "Lab" tests within PageSpeed Insights.
CrUX (Chrome User Experience Report) A dataset of real user performance data collected from millions of websites, used to populate the "Field" data section.
Key Takeaway
Google PageSpeed Insights is a diagnostic tool that combines real user data (Field) and simulated tests (Lab).
To use it effectively, enter your URL and prioritize the Core Web Vitals (LCP, INP, and CLS) found in the Field Data section.
Use the "Opportunities" list to identify specific technical fixes like optimizing images or code to improve your load times.
Fast websites respect their visitors’ time.