PageSpeed Insights

PageSpeed Insights is a free tool from Google that we use to monitor the performance of our clients' websites over time. It is one of the first things we check when taking over a new project, as within minutes it gives a quick, objective indication of the site's health. For ongoing projects, it serves as a baseline health metric.

Most people know it for the Lighthouse score, but for us, the more valuable part is the field data section at the top of the report. It comes from the Chrome User Experience Report (CrUX), which collects anonymized performance data from real Chrome users visiting the site. Unlike Lighthouse, which is a lab test that simulates a single device on a throttled connection, CrUX reflects what actual visitors experience on their own devices and networks. A good Lighthouse score doesn't guarantee a good experience for real users, and vice versa — field data gives a more accurate picture. Metrics such as LCP, INP and CLS are reported at the 75th percentile over a rolling 28-day window, separately for phone, tablet and desktop, and for both a single URL and the whole origin. These are the same numbers that Google Search uses to inform the page experience ranking factor.

The data is refreshed weekly and PageSpeed Insights keeps the history for the last 40 weeks, so it is easy to see whether performance is improving or declining after a release or a series of optimizations — without setting up any monitoring or tracking scripts. The main limitation is that a site needs enough Chrome traffic to appear in CrUX, and since the data is aggregated over 28 days, the effect of a change shows up gradually rather than immediately. PageSpeed Insights shows us where performance needs attention, but finding the root cause of a specific issue still requires dedicated debugging and profiling tools.

We used PageSpeed Insights to track the results of our performance optimisation for Christopeit Sport, where the history chart showed the shop going from failing Core Web Vitals to passing in under three months.