If you've ever run PageSpeed Insights on your website, you've seen four circular gauges showing scores from 0 to 100. These are your Lighthouse scores — a standardised measure of website quality developed by Google and used directly as a factor in search rankings.

This article explains what each score means, how it affects your business, and what actually moves the numbers.

What is Google Lighthouse?

Lighthouse is an open-source tool built into Google Chrome and available as a web service at web.dev/measure or PageSpeed Insights (pagespeed.web.dev). It runs a series of automated tests against your page and produces scores in four categories.

Scores are colour-coded: 0–49 is red (poor), 50–89 is orange (needs improvement), 90–100 is green (good). Google's official recommendation is to target the green range in all four categories.

The Four Scores

Performance

Measures how fast the page loads and becomes interactive. The Performance score is calculated from several metrics:

  • LCP (Largest Contentful Paint): how long it takes for the main content to appear. Target: under 2.5 seconds.
  • INP (Interaction to Next Paint): how quickly the page responds to user input (clicks, taps). Target: under 200ms.
  • CLS (Cumulative Layout Shift): how much elements shift around as the page loads (the jarring experience of content jumping). Target: under 0.1.
  • FCP (First Contentful Paint): when the first text or image appears. Target: under 1.8 seconds.

Performance is the score with the most direct impact on user experience and conversions. A one-second delay in load time reduces mobile conversions by approximately 20%, according to Google's research.

Accessibility

Measures how usable the website is for people with disabilities. Tests include: sufficient colour contrast between text and background, images with descriptive alt text, form fields with labels, keyboard navigation support, and correct heading hierarchy (H1, H2, H3).

A high Accessibility score doesn't just serve users with disabilities — it indicates good HTML structure that search engines can also parse more effectively. It's also a legal consideration in many jurisdictions, where web accessibility requirements apply to businesses.

Best Practices

Checks for modern web standards: HTTPS (not HTTP), no JavaScript errors in the console, images with correct aspect ratios, avoidance of deprecated APIs, and compliance with security headers.

This score is usually the easiest to bring into the green range. A site on HTTPS with no broken JavaScript and correctly sized images scores 90+ almost by default.

SEO

Checks technical SEO fundamentals: meta description present and correct length, page title present, canonical URL, robots.txt not blocking indexing, and structured data (Schema.org) without errors.

A score of 100 here doesn't guarantee rankings — it only confirms that the basic technical requirements are met. Real SEO performance also requires good content, backlinks, and Core Web Vitals (from the Performance score).

How Lighthouse Scores Affect Google Rankings

Google confirmed Core Web Vitals (LCP, INP, CLS) as ranking factors in its Page Experience update. Pages that meet the "Good" threshold in all three have a ranking advantage over pages that don't — all other factors being equal.

In practice: for keywords with high competition, a site with a Performance score of 40 will rank behind a site with comparable content but a Performance score of 90. The content quality difference has to be substantial to overcome the technical disadvantage.

What Actually Improves Performance Scores

  • Image optimisation: converting images to WebP format and compressing them typically produces the largest single improvement. Images are the most common cause of slow load times.
  • Lazy loading: loading images only when they're about to scroll into view, rather than all at once on page load.
  • Removing unused CSS and JavaScript: many WordPress themes load large CSS files where only 10–20% of the rules are used on any given page.
  • Using a CDN: a Content Delivery Network serves the site from servers geographically closer to the visitor, reducing load time.
  • Reducing third-party scripts: each analytics tag, chat widget, or social button adds load time. Audit what's actually necessary.

Realistic Score Targets by Site Type

  • Custom HTML/CSS site (no CMS): 95–100 Performance is achievable and maintainable
  • Optimised WordPress (no page builder): 85–95 is realistic with caching and image optimisation
  • WordPress with Elementor/Divi: 70–85 with significant optimisation; above 85 requires substantial technical work
  • Wix/Squarespace: 60–75 typical; limited control over optimisation

The web development services at PC Data Insights target 90+ Performance scores on all new projects, with documentation of the scores at launch. Get in touch to discuss a Lighthouse audit and improvement plan for your existing site, or to build a new site with performance built in from the start.