Key Takeaways
- Over half of mobile visitors leave a page that takes more than three seconds to load, so speed is a revenue issue, not a technical nicety.
- Lighthouse scores four areas, but Performance is the one most UK businesses fail and the one tied to Google rankings.
- Aim to pass all three Core Web Vitals: LCP under 2.5 seconds, INP under 200 milliseconds, and CLS under 0.1.
- Most low scores come from a short, fixable list: heavy images, too many scripts, no caching and slow hosting.
- Treat speed as a conversion tool, not a vanity number. Genuinely fast beats a perfect lab score.
A slow website costs you twice. Google's own research, the Need for Mobile Speed study, found that 53% of mobile visitors abandon a page that takes longer than three seconds to load. On top of that, Google ranks slow pages lower. Lighthouse is the free tool that measures the problem, and the good news is that most low scores trace back to a short, fixable list.
This guide explains what your Lighthouse score actually means, the targets to aim for, the fixes that move the needle, and how faster pages turn into better rankings and more enquiries.
What Lighthouse Actually Measures
Lighthouse, built into Chrome and Google PageSpeed Insights, scores any page from 0 to 100 across four areas:
- Performance. How fast the page loads and becomes usable. This is the one most businesses fail.
- Accessibility. How usable the page is for everyone, including people using assistive technology.
- Best Practices. Whether the code follows modern web standards.
- SEO. The basic on-page signals search engines look for.
Performance overlaps directly with Google's Core Web Vitals, which are confirmed ranking signals. That overlap is why this score matters for getting found, not just for bragging rights.
The Core Web Vitals Targets to Hit
Core Web Vitals are the three measurements Google uses to judge real-world experience. To pass, at least 75% of your real visitors need a good score on each (Google's official thresholds):
- Largest Contentful Paint (LCP), loading: good is 2.5 seconds or less. This is when your main content actually appears.
- Interaction to Next Paint (INP), responsiveness: good is 200 milliseconds or less. This is how quickly the page reacts when someone taps or clicks. INP replaced First Input Delay in 2024 and is now the most commonly failed of the three.
- Cumulative Layout Shift (CLS), stability: good is 0.1 or less. This measures how much the page jumps around as it loads, the thing that makes you tap the wrong button.
Hit all three and you have passed. In our experience auditing UK business sites, the usual culprit is a page that loads quickly on office broadband but fails on a real phone and a real mobile connection.
The Fixes That Move the Score Most
You rarely need to fix everything. These changes deliver the biggest gains, roughly in order:
- Optimise your images. Oversized images are the single biggest cause of slow pages. Compress them and serve modern formats like WebP, which can cut image weight by 80% or more.
- Reduce and defer scripts. Every extra plugin, chat widget and tracking tag blocks loading. Remove what you do not use and delay what is not needed immediately.
- Cache and use a CDN. Caching avoids rebuilding the page on every visit, and a content delivery network serves files from close to the visitor.
- Sort your fonts. Limit custom fonts and load them efficiently to avoid both delays and layout shift.
- Choose good hosting. Cheap, overloaded shared hosting puts a hard ceiling on how fast you can ever be.
Why a Faster Site Ranks and Converts Better
Google has confirmed that page experience, including Core Web Vitals, is a ranking factor. The logic is simple: fast pages keep people engaged, reduce bounce, and signal quality. So improving these scores tends to lift rankings and organic traffic.
The bigger prize is conversion. If more than half of mobile visitors leave a slow page before it loads, every second you shave off is money saved. A fast site does not just rank better, it gives more of your hard-won visitors the chance to become customers. That is why we treat speed as part of conversion work, not just technical housekeeping.
A Simple Monthly Routine
Performance drifts as you add content, images and tools. Once a month:
- Run your most important pages through Google PageSpeed Insights.
- Note your Core Web Vitals and any that have slipped from good.
- Compress any new images and remove scripts you no longer use.
- Re-test, and keep a simple log so you can see the trend.
Fifteen minutes a month protects the speed you worked to achieve.
Frequently Asked Questions
What is a good Lighthouse score? Aim for 90 or above on Performance, tested on mobile. More important than the lab number is passing Core Web Vitals on real-world data, which is what Google actually uses for ranking.
What is the difference between Lighthouse and Core Web Vitals? Lighthouse is a lab test that simulates a page load and gives a score out of 100. Core Web Vitals are the real-world measurements from actual Chrome users. Lighthouse helps you find and fix problems; Core Web Vitals are what count for SEO.
Does improving my Lighthouse score really help SEO? Yes, reliably if indirectly. Speed is a confirmed ranking signal, and faster pages improve engagement, which supports rankings further. It is rarely the single biggest factor, but it is one of the few fully within your control.
Why is my score high on desktop but low on mobile? The mobile test deliberately simulates a slower device and connection. Since most visitors are on mobile, the mobile score is the one to prioritise.
How fast should my website load? Aim for your main content to appear within 2.5 seconds on a mobile connection, in line with the LCP target. Under three seconds overall is the threshold where you stop losing the majority of impatient mobile visitors.
The Bottom Line
Your Lighthouse score is a proxy for something that matters to your bottom line: a fast, smooth website that Google and your customers both prefer. Focus on images, scripts, caching and hosting, aim to pass all three Core Web Vitals, and treat speed as a conversion tool rather than a trophy.
If your site is slow and you want it genuinely fast, measured on real phones and real connections, get in touch. We offer website speed optimisation for UK businesses.
