What is Google Lighthouse?
A plain English guide to the tool that grades your website - and why the scores affect where you appear on Google.
Get a free site auditWhat is Google Lighthouse?
Imagine Google sends an inspector to visit your website. The inspector checks everything - how fast it loads, how easy it is to use, whether it follows web standards, and how well it is set up for search engines. At the end of the visit, they hand you a report card with scores out of 100 in four categories.
That inspector is Google Lighthouse. It is a free, automated tool built by Google itself, and it is the closest thing to an official measure of website quality that exists.
Every website can be tested with Lighthouse. The scores are public and repeatable. When I say a website scores 100 on Lighthouse, I mean it achieved a perfect score on that report card - across all four categories.
The four scores explained
Lighthouse grades your website in four areas. Each is scored from 0 to 100.
Performance
How fast your website loads for a real visitor. This is the score that has the most direct impact on your Google ranking and on whether visitors stay or leave.
Measures: loading speed, responsiveness, visual stability
Accessibility
How easy your website is to use for everyone, including people with disabilities who use screen readers, keyboard navigation, or other assistive tools.
Measures: contrast, labels, keyboard usability, alt text
Best Practices
Whether your website follows modern web standards - things like using secure connections (HTTPS), not running outdated code, and avoiding known security vulnerabilities.
Measures: security, modern standards, error-free code
SEO
Whether your website is set up correctly for search engines to find, read, and understand it. This covers things like page titles, descriptions, mobile-friendliness, and structured content.
Measures: meta tags, mobile, crawlability, structured data
What are Core Web Vitals?
Core Web Vitals are three specific measurements that sit inside the Performance score. Google announced in 2021 that they are an official ranking factor - meaning they directly affect where your website appears in search results.
The three measurements sound technical, but each one describes something a real visitor would notice:
Largest Contentful Paint (LCP)
In plain English: how long does it take before the main thing on the page appears?
Think of it as how long a visitor stares at a blank or half-loaded screen before they can see what the page is actually about. Google wants this under 2.5 seconds. Above 4 seconds is considered poor.
Common culprit: large uncompressed images, slow hosting
Interaction to Next Paint (INP)
In plain English: when a visitor clicks something, how quickly does the page respond?
If someone clicks a menu, fills in a form, or taps a button and the page feels sluggish or frozen for a moment, that is a poor INP score. Google wants a response within 200 milliseconds - essentially instant.
Common culprit: too many scripts running at once, bloated plugins
Cumulative Layout Shift (CLS)
In plain English: does anything jump around on screen while the page is loading?
You have experienced this - you start reading something or go to click a button, and suddenly an image or advert loads and pushes everything down the page. That is a layout shift, and it is frustrating. Google measures how much this happens. The target is a score as close to zero as possible.
Common culprit: images without dimensions, late-loading fonts or ads
Why does this affect your business?
There are two reasons to care about Lighthouse scores and Core Web Vitals.
1. Google uses them to decide where you rank
Google's job is to send people to the best possible result for their search. A slow, hard-to-use website is not the best result. Since 2021, Google has used Core Web Vitals as a direct ranking signal - a fast website has a concrete advantage over a slow one, all else being equal.
For local businesses competing for search terms like "WordPress developer Surrey" or "plumber Caterham", where the difference between two businesses can be small, page speed can be the tiebreaker.
2. Slow websites lose customers directly
The research on this is consistent:
- 53% of mobile visitors leave a page that takes longer than 3 seconds to load (Google, 2018)
- A one-second delay in page load time reduces conversions by an average of 7% (Akamai)
- Pages that load in 1 second convert 3x better than pages that load in 5 seconds (Portent)
A visitor who cannot see your page quickly enough will hit the back button and call your competitor instead. You paid for that visitor through SEO or advertising and never got a chance to talk to them.
What causes low Lighthouse scores?
Most low scores come from the same handful of problems - and nearly all of them trace back to how the website was built rather than anything the business owner did wrong.
Page builder bloat
Tools like Divi, Elementor, and WPBakery are popular because they make it easy to build websites visually. The trade-off is they load a large amount of code that your visitors do not need - slowing down every single page.
Too many plugins
WordPress plugins are useful, but every plugin adds code that the browser has to download and process before your page appears. A website with 30+ plugins - which is common - can be significantly slower than one built lean with only what it needs.
Unoptimised images
A photo taken on a phone or downloaded from a stock site can be 5-10MB. Uploaded directly to a website without compression, it forces every visitor to download that entire file before they can see your page. Modern image formats like AVIF and WebP can reduce that to under 200KB with no visible difference in quality.
Cheap or shared hosting
Budget hosting puts hundreds of websites on one server. When another website on the same server gets busy, yours slows down. Hosting is one of the biggest factors in how fast a website feels - and it is often the last thing people think about.
No caching or CDN
Without caching, your server builds the page from scratch every single time someone visits it. A CDN (Content Delivery Network) stores copies of your site closer to where visitors are located. Both are standard techniques that most budget websites skip.
Render-blocking scripts
Some scripts - analytics, chat widgets, advertising - are loaded in a way that pauses your entire page while they download. The visitor sees a blank screen. Loading these scripts in the background instead is a straightforward fix that many developers do not apply.
What does a score of 100 actually mean?
A Lighthouse score of 100 means the tool found nothing to criticise. The page loads as fast as it practically can, follows all modern standards, is accessible to every visitor, and is set up correctly for search engines.
It does not mean the website is perfect in every way - design, content, and conversion strategy matter too. But it does mean the technical foundation is as strong as it can be. A competitor with a score of 45 is carrying unnecessary weight that costs them in both rankings and customer conversions every single day.
Every website I build scores 100 across all four Lighthouse categories. That is not a marketing claim - it is a measurable, verifiable result that you can check yourself the day your site goes live using Google's own free tool.
Want to see how your current website scores? You can test any website for free at pagespeed.web.dev - just enter your URL. If any category scores below 90, there is room for improvement.
Frequently asked questions
Q: My website looks fine to me - does the score really matter?
A: It depends on your connection speed and device. A fast fibre connection and a modern laptop will make almost any website feel fast. But a significant proportion of your visitors are on mobile data, older phones, or slower connections - and those visitors experience your website very differently. Lighthouse tests under realistic conditions, not ideal ones.
Q: Can I check my own Lighthouse score?
A: Yes. Go to pagespeed.web.dev, enter your website address, and Google will run a full Lighthouse report in about 30 seconds. The results are colour-coded: green (90-100) is good, orange (50-89) needs improvement, red (0-49) is poor.
Q: My web developer said my site is optimised - but the score is 55. Who is right?
A: The score. Lighthouse is Google's own tool and the result is objective. A score of 55 means there are specific, identified problems that Google considers significant. A developer saying a site is "optimised" without being able to show you a Lighthouse score is not using a measurable definition of the word.
Q: How much difference does it actually make to my Google ranking?
A: Core Web Vitals are one of around 200 factors Google uses to rank pages. They are not the only factor, and a website with exceptional content and hundreds of quality backlinks can still rank well despite poor performance scores. But for local businesses competing on similar terms with similar content, page speed is increasingly the differentiator - particularly for mobile searches, which now account for the majority of Google traffic.
Q: Is fixing a slow WordPress site expensive?
A: It depends on the cause. Simple fixes - image compression, plugin cleanup, caching configuration - can often improve a score significantly in a few hours. More complex issues like replacing a heavy page builder require more work. I offer a free initial review so you know exactly what is causing the problem and what fixing it would involve before committing to anything.
Q: Do you guarantee a Lighthouse score of 100?
A: On new builds, yes - every website I build achieves 100 across all four categories. On existing site optimisation projects, the target is always 90+ across all categories, with 100 as the goal. The exact outcome depends on constraints like existing hosting and third-party integrations (such as live chat or advertising scripts) that are outside the core website.
How I fix these problems
When I audit a website I run it through Lighthouse and trace every low score back to its root cause. The fix might be compressing images, removing unused plugins, switching to faster hosting, or replacing a bloated page builder with clean custom code. Every site I build starts from scratch with performance as the primary constraint - not an afterthought applied at the end.
If your existing WordPress site is slow, I offer a free initial review that tells you exactly what is dragging your scores down and what it would take to fix it. You also might find my guide on responsive web design useful - performance and mobile-friendliness go hand in hand.
Find out how your site scores
I will run a full Lighthouse audit on your website and send you a plain English summary of what the scores mean and what, if anything, needs fixing.
Get a free site audit