What a Lighthouse Score Actually Means for Your Website

We include a Lighthouse audit in every project we deliver, and we require a score above 90 in Performance, Accessibility, and Best Practices before a site goes live. Clients sometimes ask what these numbers actually mean and whether they matter outside of a technical conversation. They do, and this is a plain account of why.

What Lighthouse measures ¶

Lighthouse is an open-source tool built into Chrome's developer tools. It audits a web page across four categories: Performance, Accessibility, Best Practices, and SEO. Each category produces a score from 0 to 100. The scores are not arbitrary. They are calculated from specific, documented metrics that correlate with real user experience.

Performance is the most consequential for most business sites. It measures how quickly the page loads and becomes interactive for a real user on a real connection. The specific metrics include Largest Contentful Paint (how long until the main content is visible), Total Blocking Time (how long the page is unresponsive to input), and Cumulative Layout Shift (how much the page jumps around as it loads).

Why a low performance score costs you visitors ¶

Google's own research found that the probability of a visitor leaving a page increases significantly as load time increases. A page that takes five seconds to load loses a meaningful proportion of visitors before they have seen anything. For a business that pays for traffic through ads or SEO, a slow site is a direct cost.

A Lighthouse Performance score below 50 usually indicates one or more of the following: images that have not been compressed or converted to a modern format, JavaScript that blocks the page from rendering, or a hosting environment that responds slowly. All of these are fixable, and most of them are fixable without a full rebuild.

Accessibility is not optional ¶

The Accessibility score measures whether the site can be used by people with visual, motor, or cognitive impairments. In the UK, the Equality Act 2010 applies to websites, and a site that is not reasonably accessible to disabled users carries legal risk. Beyond the legal question, an inaccessible site is simply a site that excludes a portion of your potential audience.

Common accessibility failures that Lighthouse catches include images without alt text, form fields without labels, insufficient colour contrast between text and background, and interactive elements that cannot be reached by keyboard. None of these are difficult to fix if they are caught early. They are expensive to fix if they are discovered after launch.

If you want to know where your current site sits, run it through PageSpeed Insights, which uses the same engine as Lighthouse and is free. The results page will tell you exactly what is pulling the score down and roughly how much each fix would improve it.