What comes back looks like a school report card: five scores, mostly out of 100, in colours ranging from “well done” green to “see me after class” red. Most business owners glance at the numbers, feel a bit anxious, and close the tab. This guide is so you don’t have to do that. I’ll walk through what each section actually means, what’s realistically fixable yourself, and where you’d need a developer.
There are five categories now, not four; Google quietly added a new one, Agentic Browsing, in May 2026. More on that below, because it’s genuinely new territory and most sites are failing it without knowing it exists.
1. Performance
This is the one everyone fixates on, and it’s the most technical. It measures how fast your page loads and how quickly it becomes usable, scored against Google’s Core Web Vitals: Largest Contentful Paint (how fast the main content appears), Cumulative Layout Shift (whether things jump around while loading), and Interaction to Next Paint (how snappy the site feels when clicked).
Common causes of a low score: unoptimised images (the single biggest culprit on most small business sites), too many plugins loading scripts on every page, no caching, cheap hosting, and render-blocking CSS or JavaScript that stalls the page before anything shows up.
What you can do yourself: compress images before uploading (TinyPNG or Squoosh), deactivate plugins you don’t use, and check if your host offers any caching layer. Google’s own Core Web Vitals documentation is worth a read if you want the detail behind each metric.
What usually needs a developer: critical CSS delivery, deferring non-essential scripts, fixing layout shifts caused by fonts or ads loading late, and diagnosing server response time. This is exactly the kind of work I do day to day; it’s not glamorous, but it’s the difference between a 45 and a 95. It’s covered under my web design & development service.
2. Accessibility
This checks whether people using screen readers, keyboard navigation, or assistive tech can actually use your site. It’s scored on things like images having alt text, sufficient colour contrast between text and background, form fields having proper labels, and buttons/links being usable without a mouse.
This category matters for three reasons beyond “it’s the right thing to do”: it’s a legal requirement under the Equality Act for many UK businesses (particularly public sector and healthcare); it’s a growing factor in how Google evaluates page quality; and, this is the bit people miss, a genuinely accessible site tends to be a better-structured site full stop, which feeds directly into the SEO and Agentic Browsing scores below.
What you can do yourself: add alt text to images and check your colour contrast with a free tool like WebAIM’s contrast checker.
What usually needs a developer: fixing ARIA roles, form label associations, focus order, and semantic HTML structure. I offer accessibility audits specifically because this is a category most agencies gloss over – it’s a genuine gap in the market and worth mentioning to clients in healthcare, legal, and public sector work, especially.
3. Best Practices
This is a catch-all for modern web standards: whether you’re serving your site over HTTPS, whether your images have the right aspect ratio set (to avoid layout shift), whether you’re using deprecated APIs, and whether there are any browser console errors.
It’s usually the easiest category to fix and the one clients are most surprised to see flagged, because it often catches small technical debt: an old plugin doing something outdated, a missing security header, or a leftover script from a redesign three years ago nobody removed.
What you can do yourself: not much beyond checking your SSL certificate is valid and current.
What usually needs a developer: everything else here is code-level housekeeping; a good one to bundle into a maintenance package rather than a one-off fix, since these issues tend to creep back in as plugins update.
4. SEO
This checks the technical foundations that let Google (and now AI Search) understand and rank your page: title tags, meta descriptions, whether your content is crawlable, mobile-friendliness, and structured data (schema markup).
Important caveat: this score is a technical floor, not a ranking guarantee. Scoring 100 here means Google can read your page properly; it says nothing about whether your content is good enough to rank. Don’t let a green SEO score lull you into thinking the content work is done.
What you can do yourself: write proper meta titles and descriptions for your key pages and make sure every page has one clear heading structure.
What usually needs a developer: structured data (LocalBusiness schema, product schema, and FAQ schema), fixing crawlability issues, and technical redirects. This is foundational work I build into every site I create, because retrofitting it later is always more expensive than doing it right the first time.
5. Agentic Browsing (the new one)
This category only started appearing in PageSpeed Insights reports from around May 2026, so if you tested your site before then, run it again; you’ll see something new.
It’s different from the other four in a key way: instead of a score out of 100, you get a pass ratio, like 3/3. It’s checking whether AI agents – the tools people are increasingly using to browse and act on the web on their behalf, like ChatGPT’s browsing mode or Google’s Project Mariner – can actually navigate your site and do something useful, not just read it.
The three default checks are:
- A clean accessibility tree. This is the same structural information used for screen readers, so if your accessibility score is solid, you’re most of the way there already.
- Low Cumulative Layout Shift. If your layout stays stable for humans, it stays stable for agents too.
- A valid
llms.txtfile at your domain root. This is a plain-text file that tells AI tools what your site is and how to interpret it; it’s genuinely new and something almost no small business sites have yet.
Google itself flags this whole category as “experimental” and “under development”, so a low score here is not an emergency the way a red performance score is. But it’s worth taking seriously early rather than late – this is the same position Core Web Vitals were in back in 2020 before they became something every business needed to care about. Getting your accessibility and layout stability right (which you should be doing anyway) covers two of the three checks. The llms.txt file is a genuinely quick win; it costs almost nothing to add and puts you ahead of nearly every competitor in your area who hasn’t heard of this yet.
A quick note on Lighthouse in Chrome DevTools
PageSpeed Insights and the Lighthouse panel in Chrome DevTools are the same engine under the bonnet; Lighthouse is what actually runs the audit; PageSpeed Insights is Google’s hosted version of it with some extra real-world data layered on top.
To run it yourself: open any page in Chrome, press F12 (or right-click → Inspect), go to the “Lighthouse” tab, choose your categories and device type, and click “Analyse page load”. The advantage over PageSpeed Insights is that you can test pages behind a login, test a staging site before it goes live, and throttle the network/CPU manually to see how the site behaves on a slower connection or older phone.
The trade-off: results from DevTools will often differ slightly from PageSpeed Insights because they’re run on your own machine’s hardware and network rather than Google’s servers. Neither is “wrong”; they’re just different test conditions. If you want a consistent baseline for tracking progress over time, stick to one tool and one method.
Alternatives worth knowing about
PageSpeed Insights is free and a good starting point, but it’s not the only tool, and each does something slightly different:
- GTmetrix — Runs on the same Lighthouse engine but gives you a proper waterfall chart, a filmstrip of the page loading frame by frame, and the ability to choose your test server location. Good for a more visual, presentable report if you’re showing a client or stakeholder what’s actually happening.
- WebPageTest — The most technical and granular of the bunch. Lets you test from dozens of global locations, on real devices, over throttled connections. Overkill for a quick check, but brilliant if you’re diagnosing something specific like a slow server response.
- Pingdom — Similar territory to GTmetrix, with a strong focus on uptime monitoring alongside speed testing if you want both in one place.
My honest take: for a single spot-check, PageSpeed Insights is fine. If you want to track a site’s health over months or need a report that makes sense to a non-technical client, GTmetrix is worth the extra step.
What to actually do with your results
Running the test is the easy 5% of this. Reading five categories full of technical audits and knowing which ones are worth fixing, which can wait, and which need a developer’s hands in the code — that’s the part that eats people’s Saturday afternoons.
If you’ve run your site through PageSpeed Insights and come out the other side with a page full of red and orange, that’s exactly the kind of thing I look at for clients regularly. I offer full site audits covering performance, accessibility, best practices, SEO and now agentic browsing readiness, with a clear, prioritised list of what’s worth fixing first – no jargon, no upsell padding, just what will actually move the needle for your business.
Get in touch via my contact page if you’d like your site properly looked at.
