Website speed and Core Web Vitals: a practical guide

#The Core Web Vitals at a glance

Google assesses these values on real visits over the last 28 days, at the 75th percentile: at least three visits out of four must be "good". Mobile and desktop are assessed separately. INP replaced the old FID metric on 12 March 2024.

#What hurts LCP

• images that are too heavy, wrongly sized or in an old format;
• a slow server response;
• CSS or JavaScript that blocks rendering;
• a main image that loads too late (lazy-loaded, or discovered late by the browser).

#What hurts INP

• heavy JavaScript keeping the browser busy;
• too many third-party scripts: chat, ad pixels, widgets;
• click handlers that do a lot of work before showing anything.

#What hurts CLS

• images, videos or embeds with no reserved dimensions;
• banners and ads inserted above the content;
• web fonts that change the text size as they load.

#How much does it count for SEO?

Google uses the Core Web Vitals in its ranking systems, as part of what it calls page experience. But it says so itself: good scores don't guarantee a good position, and relevant content remains the main criterion. Between two equally useful pages, the faster one has the edge.

A useful clarification, because a lot gets written on this: the March 2026 core update (rolled out from 27 March to 8 April) aimed, according to Google, to better surface relevant, satisfying content. Google announced no change to the weight of the Core Web Vitals at that time.

The real stake lies elsewhere: a visitor who has to wait, or whose button jumps just as they tap it, leaves. Speed serves your conversions first.

#How to measure your website's speed

• PageSpeed Insights (pagespeed.web.dev): field data from your visitors (if your site has enough traffic) and a lab test with recommendations. "No data" simply means Google doesn't yet have enough visits to produce a field measurement.
• Google Search Console, Core Web Vitals report: your pages grouped as "good", "need improvement" and "poor", on mobile and desktop.
• Chrome DevTools: the Performance panel shows the Core Web Vitals live as you browse, and Lighthouse runs a full audit.

Always test on a mid-range phone and a mobile connection, not just on your office computer.

#10 concrete ways to speed up your website

#1. Optimise your images

• Use the AVIF or WebP formats, now supported by all recent browsers.
• Serve images at the right size for each screen.
• Compress them (Squoosh, TinyPNG or your CMS's built-in optimisation).
• Lazy-load (loading="lazy") images below the fold, never the main image, which should instead get fetchpriority="high".

<img src="/hero.avif" width="1200" height="800" alt="Joinery workshop in Schaerbeek" fetchpriority="high">
<img src="/project.avif" width="800" height="600" alt="Kitchen fitted in Uccle" loading="lazy">

#2. Choose fast hosting

Server response time is the foundation. Choose a host with servers in Europe, HTTP/2 or HTTP/3, server-side caching and a content delivery network (CDN) for static files.

#3. Trim CSS and JavaScript

#4. Preload critical resources

Tell the browser what it needs first: the main image and the body text font.

<link rel="preload" as="image" href="/hero.avif" fetchpriority="high">
<link rel="preload" as="font" href="/fonts/body.woff2" type="font/woff2" crossorigin>

#5. Reserve space for elements

#6. Look after your fonts

#7. Limit third-party scripts

Every external tool (chat, pixels, maps, videos, reviews) has a cost. Take stock, remove what you no longer use, and load the rest after an interaction or after cookie consent.

#8. Turn on caching

Static files whose names change with every release can be cached for a year:

Cache-Control: public, max-age=31536000, immutable

#9. Break up long tasks

For INP, avoid a click triggering a long computation before anything is shown. Show a response first (button pressed, loading indicator), then do the work.

#10. Keep monitoring

Check PageSpeed Insights and the Search Console report every month, and after every theme, plugin or major content update.

#Common mistakes

• A carousel at the top of the page: heavy to load and rarely watched beyond the first slide.
• An autoplaying video: use an image with a play button instead.
• Lazy-loading the main image: that image should arrive first.
• Piling up WordPress plugins: each one adds its own CSS and JavaScript.
• Pop-ups that shift the page or block the screen on mobile.
• Testing only on desktop over fibre.

#WordPress, Shopify, custom: what can you optimise?

• WordPress: a lightweight theme, a caching plugin, automatic image optimisation and as few plugins as possible are often enough to turn green.
• Shopify: the choice of theme and the number of installed apps matter most; every app adds scripts.
• Custom development: speed is built in from the start, with pages generated in advance and images optimised automatically. That is how leadco.be is built, with Next.js.

#Frequently asked questions

#Are the Core Web Vitals really a Google ranking factor?

Yes, Google has used them since 2021, but their weight is limited compared with content relevance. They mostly make the difference between pages of similar quality, and on your conversions.

#My Lighthouse score is good but Search Console says otherwise: why?

Lighthouse tests the page once, under simulated conditions. Search Console reflects your real visitors over 28 days, with their phones and connections. Field data is what counts for Google.

#How long before improvements show?

Lab tests change immediately. Field data updates gradually: allow about four weeks for Search Console to reflect your fixes.

#Why does PageSpeed Insights show "No data" for my site?

Because Google hasn't yet collected enough real visits for your page or site. Rely on the lab test and the Search Console report, which will fill up as traffic grows.

#Is WordPress too slow for good Core Web Vitals?

No. A well-configured WordPress site (lightweight theme, caching, optimised images, few plugins) can pass all three thresholds. Heavy themes and piles of plugins are what slow it down.

#Should I aim for 100 out of 100?

No. The Lighthouse score is a diagnostic tool. The goal is three green metrics for your real visitors.