What the website speed test measures
Lighthouse measures how a page behaves in a controlled test environment and produces a performance report from several loading and rendering metrics. It can also identify opportunities such as inefficient image delivery, render-blocking resources, excessive JavaScript work and other work that delays the page.
The headline performance score is useful for comparison, but the diagnostics underneath it normally tell you more about what needs fixing.
Core Web Vitals need real-user data too
Google's current Core Web Vitals are Largest Contentful Paint, Interaction to Next Paint and Cumulative Layout Shift. They measure loading performance, responsiveness and visual stability. Google's recommended good thresholds are LCP within 2.5 seconds, INP of 200 milliseconds or less and CLS of 0.1 or less, assessed at the 75th percentile of visits.
A Lighthouse test is laboratory data. Core Web Vitals reported from Chrome user data or Google Search Console are field data based on real visits, so the two can differ. Use the test to diagnose technical causes, but check field data when deciding whether real users are experiencing a persistent problem.
What commonly makes a website slow
Performance problems are rarely solved by installing a single optimisation plugin. Large hero images can delay the main visible content. Third-party scripts can occupy the main thread. Fonts and CSS can block rendering. Poor caching or slow server responses can delay every request before the browser has much useful work to do.
That is why two pages on the same website can score very differently. Test the actual page that matters, particularly templates with large media, ecommerce functionality or third-party tracking and widgets.
What to do with a poor speed score
Start with the largest opportunities in the report and retest after each meaningful change. Do not remove necessary functionality simply to chase a perfect score. Google states that Core Web Vitals contribute to its assessment of page experience, but good scores alone do not guarantee rankings.
For a one-off technical optimisation, see our Website Speed Fix. It includes a performance audit, image, script, font and caching work, hosting-level tuning where possible, and before and after reporting. Ongoing monitoring is available through the Speed Retainer at £89 per month or £890 per year.