You ran a Lighthouse report on your website. Maybe someone told you to, maybe you stumbled across it in Chrome DevTools, maybe you used Google’s free PageSpeed Insights tool and suddenly found yourself staring at four colored circles with numbers inside them.
One of them is green. One is orange. One is red. You’re not sure what any of it means or what you’re supposed to do about it.
This post is for you.
What Is Google Lighthouse?
Lighthouse is Google’s open-source auditing tool. It runs a series of automated tests against any webpage and returns scores across four categories: Performance, Accessibility, Best Practices, and SEO.
It’s built into Chrome DevTools, but most people access it through Google PageSpeed Insights — which runs the same Lighthouse tests from Google’s servers and layers in real-world field data on top.
The scores run from 0 to 100. The color coding is:
- Green (90–100): Good. Google considers this passing.
- Orange (50–89): Needs attention. Room for improvement.
- Red (0–49): Problem. This is actively hurting you.
The goal — and what I guarantee on every site I build through the Straight-A Guarantee — is green across all four.
Here’s what each category is actually measuring.
Performance: The Score Everyone Obsesses Over
The Performance score measures how quickly your page loads and how smoothly it delivers content to visitors. It’s calculated from a set of specific timing metrics, each weighted differently.
The most important ones to understand:
Largest Contentful Paint (LCP) — How long it takes for the main content of the page to appear. For most sites, this is your hero image or the largest block of text above the fold. Google considers anything under 2.5 seconds good. Over 4 seconds is poor.
Total Blocking Time (TBT) — How long the browser is “blocked” from responding to user input while scripts execute. High TBT means your page appears to load but then freezes for a moment before you can interact with it. Users notice this even if they can’t name it.
Cumulative Layout Shift (CLS) — Whether elements move around while the page loads. If you’ve ever tried to click a button and had it jump out of the way at the last second, that’s a high CLS score in action. It’s deeply frustrating and Lighthouse penalizes it directly.
Speed Index — A visual measure of how quickly content appears above the fold during loading. Not the same as when the page technically “finishes” loading — it measures how fast it feels.
One important nuance: the mobile score and desktop score are different, and the mobile score is the one that matters for SEO. Google uses mobile-first indexing, so a site that scores 95 on desktop but 42 on mobile is still a slow site in Google’s eyes. Always check mobile first.
A low Performance score almost always traces back to one of these causes: a heavy page builder generating bloated code, unoptimized images, poor hosting, or missing caching. I covered all of these in detail in What Is Website Performance? (And Why Most WordPress Sites Get It Wrong) — if your Performance score is in the red, that’s the place to start.
Accessibility: More Than a Legal Checkbox
The Accessibility score measures how usable your site is for people with disabilities — those using screen readers, keyboard navigation, voice control, or other assistive technologies.
Lighthouse checks things like:
- Do all images have descriptive alt text?
- Is the color contrast between text and background sufficient?
- Do form elements have proper labels?
- Can all interactive elements be reached and activated with a keyboard?
- Are heading levels used in a logical, sequential order?
- Does the page have a proper language attribute set?
A common misconception is that accessibility is only relevant for organizations legally required to comply with ADA or WCAG standards. That’s not true. Accessibility improvements almost always improve the experience for everyone — clearer contrast helps people reading in bright sunlight, logical heading structure helps users scanning for information quickly, and proper alt text feeds directly into image SEO.
The legal dimension is real though. The DOJ has issued guidance making clear that websites are subject to ADA accessibility requirements, and lawsuits over inaccessible websites are not uncommon. Getting your Accessibility score into the green is both the right thing to do and smart risk management.
A score below 70 here typically means there are systematic issues — not just a missing alt tag here and there, but structural problems with how the site was built. Themes that use poor color contrast by default, forms without labels, or non-semantic HTML are common culprits.
Best Practices: Technical Hygiene
Best Practices is the least glamorous of the four scores, but it’s a useful indicator of overall technical health. It checks whether your site follows modern web development standards.
Common items that affect this score:
- HTTPS — Is the site served securely? Any page still serving over HTTP will take a hit here.
- Image formats — Are you using modern formats (WebP, AVIF) instead of outdated ones?
- Browser errors — Are there JavaScript errors showing up in the browser console?
- Deprecated APIs — Is the site using outdated browser features that are being phased out?
- Third-party cookies — As browsers phase out third-party cookies, sites still relying on them will be flagged.
- Safe external links — Do links opening in a new tab use
rel="noopener"to prevent security vulnerabilities?
A Best Practices score below 90 often points to something in your plugin stack or theme code — outdated JavaScript libraries, insecure external scripts, or console errors thrown by a poorly maintained plugin. These can be tricky to trace without knowing what you’re looking for in the diagnostics.
SEO: On-Page Signals
The SEO score checks the basic on-page elements that help search engines crawl, understand, and rank your content. This is not a comprehensive SEO audit — it’s a technical foundation check.
Things Lighthouse evaluates here:
- Meta description — Does the page have one? Is it a reasonable length?
- Title tag — Does the page have a unique, descriptive title?
- Crawlability — Is the page blocked from indexing by robots.txt or a noindex tag? (This has caught more than one client by surprise.)
- Mobile friendliness — Is text legible without zooming? Are tap targets large enough?
- Structured data — If you’re using schema markup, does it validate correctly?
- Links have descriptive text — “Click here” and “read more” are flagged as poor anchor text.
A low SEO score here doesn’t necessarily mean your page won’t rank — there are hundreds of ranking factors Lighthouse doesn’t touch. But it does mean there are technical hygiene issues that are making Google’s job harder. Fixing these is table stakes.
How to Actually Use Your Results
Knowing what the categories measure is one thing. Knowing what to do with a specific score is another.
If your Performance score is below 50 on mobile: This is almost always a structural problem — a heavy page builder, poor hosting, or both. Optimization plugins can improve it somewhat, but there’s a ceiling. At this point it’s worth reading the honest breakdown of what causes WordPress sites to be slow and assessing whether the foundation needs to change.
If your Performance score is 50–75: You’re in the orange. Targeted fixes — image optimization, caching, a hosting upgrade — can likely push you into the green without a rebuild. Run the full PageSpeed Insights report and work through the Opportunities section from highest to lowest time savings.
If your Accessibility score is below 70: There are systematic issues, not just a few missing alt tags. A full accessibility audit would tell you exactly what needs fixing, but the most common issues are color contrast, missing form labels, and non-semantic heading structure.
If your Best Practices score is below 90: Open Chrome DevTools, go to the Console tab, and reload the page. Most Best Practices failures leave errors there. Often it’s a single outdated plugin throwing JavaScript errors across every page.
If your SEO score is below 90: Check for a missing or duplicate meta description first — that alone drops the score significantly. Then check that the page isn’t accidentally set to noindex.
One Thing Lighthouse Doesn’t Tell You
Lighthouse is a lab test. It simulates a page load under controlled conditions, which is useful for consistency but doesn’t always reflect what real users experience.
For sites with enough traffic, PageSpeed Insights also shows Core Web Vitals field data — actual measurements from real Chrome users visiting your site. This is the data Google uses for its ranking signals, not the lab score.
If your lab score looks decent but your field data shows “Poor” for LCP or CLS, that gap is important. Real users are having a worse experience than the simulation suggests — often because of third-party scripts (ads, chat widgets, video embeds) that don’t show up cleanly in lab tests.
For a full breakdown of Core Web Vitals and what each metric means in plain English, the non-techie’s guide to Core Web Vitals covers all of it.
Frequently Asked Questions
What’s a good Lighthouse score? 90 or above in all four categories is the target. That’s the green range, and it’s what I guarantee on every site I build. Anything below 50 in any category is worth addressing promptly.
Does my Lighthouse score affect SEO? Performance does, directly — Google uses Core Web Vitals (which feed into the Performance score) as a ranking signal. The SEO score flags technical issues that affect crawlability and indexability. Accessibility and Best Practices don’t directly affect rankings but affect user experience, which affects engagement signals.
Why do I get different scores each time I run Lighthouse? Lighthouse scores have some variability, especially for Performance, because network conditions and server response times fluctuate. Running three tests and averaging them gives a more reliable number than relying on a single result.
My desktop score is great but my mobile score is terrible — which matters more? Mobile. Google uses mobile-first indexing, so your mobile score is the one that influences rankings. A high desktop score with a low mobile score is a common pattern on sites with heavy themes or page builders that haven’t been optimized for mobile performance.
Can I improve my score without rebuilding my site? Sometimes. Image optimization, caching, and a hosting upgrade can push an orange score into the green. But if your Performance score is below 50 on mobile and those fixes haven’t moved it, the problem is usually architectural — the way the site was built — and optimization will only take you so far. That’s when a performance-first rebuild is worth considering.
What’s the difference between Lighthouse and PageSpeed Insights? PageSpeed Insights uses Lighthouse under the hood but adds real-world field data from Chrome users on top. For most purposes they’re the same tool — PageSpeed Insights is just the more accessible version and includes the field data layer, which is more meaningful for SEO purposes.
The Bottom Line
A Lighthouse report is a snapshot of your site’s technical health across four dimensions. Green across all four means your site is fast, accessible, technically sound, and properly set up for search engines. Red in any category means there’s work to do.
The Performance score gets the most attention because it has the most direct business impact — slow sites lose visitors and rank lower. But all four scores matter, and a genuinely well-built site earns green in all of them.
If you’re looking at a red Performance score and wondering whether it’s fixable or whether it’s a sign of something deeper, I’m happy to take a look and give you a straight answer.
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