Core Web Vitals

What are the Core Web Vitals?

The Core Web Vitals are three web performance metrics that Google defines as essential to a good user experience. Since June 2021 they have been part of Google's ranking factors and they directly influence where your site appears in the search results. In 2024, Google updated them by replacing FID with INP.

They cover three aspects of the experience: loading speed (LCP), responsiveness (INP) and visual stability (CLS). Each metric has a good, a needs improvement and a poor threshold.

LCP (Largest Contentful Paint)

LCP measures how long it takes to display the largest visible element on the page (hero image, main block of text, video). It reflects how fast the page feels to the user. The thresholds are: good under 2.5 seconds, needs improvement from 2.5 to 4 seconds, poor beyond 4 seconds.

To improve LCP: optimise your images (WebP/AVIF format, lazy loading, srcset), use a CDN, preload critical resources, keep blocking CSS and JavaScript to a minimum, and use server-side rendering (SSR). With Next.js and its optimised Image component, an LCP under 2 seconds is standard.

INP (Interaction to Next Paint)

INP (which has replaced FID since March 2024) measures the overall responsiveness of the page: the time between the user's interaction (click, tap, keystroke) and the visual response of the page. The thresholds are: good under 200ms, needs improvement from 200 to 500ms, poor beyond 500ms.

INP is tougher than the old FID because it measures every interaction during the session, not only the first one. To improve it: reduce the JavaScript running on the main thread, break up long tasks (anything over 50ms), use web workers for heavy computation, and avoid massive JavaScript hydration on load.

CLS (Cumulative Layout Shift)

CLS measures visual stability: the unexpected movement of elements during loading. Ever clicked a button only for an image to load at that exact moment and push the button further down? That is a layout shift. The thresholds are: good under 0.1, needs improvement from 0.1 to 0.25, poor beyond 0.25.

To improve CLS: always set the dimensions (width, height) of images and videos, reserve space for ads and iframes, use font-display: swap for your fonts, and avoid inserting content dynamically above existing content.

The impact on SEO and on the business

Google has confirmed that the Core Web Vitals are a ranking factor. With equal content, the better performing page wins. According to a Vodafone study, improving LCP by 31% increased sales by 8%. Amazon estimates that 100ms of extra latency costs 1% of sales.

Beyond SEO, the Core Web Vitals directly affect the user experience and your conversion rate. A fast, stable site inspires trust and reduces drop-off.

How to measure your Core Web Vitals

Use PageSpeed Insights (pagespeed.web.dev) for a quick diagnosis. Google Search Console shows the Core Web Vitals for all of your pages in the Experience section. Chrome DevTools (the Performance tab) allows a detailed analysis. Lighthouse produces a full report with recommendations.

Lab data or field data: which one counts?

One essential distinction escapes most site owners. Tools like Lighthouse produce lab data: a simulation under controlled conditions, useful for debugging. But Google takes field data into account, collected from real users through the CrUX report (Chrome User Experience Report).

In practice, your Lighthouse score can be excellent while your field data stays poor, because your real visitors are on entry level smartphones over slow 4G networks. For a tradesperson's brochure site whose customers mostly browse on the move, it is the field data that decides the ranking. Always base your decisions on the "Discover what your real users are experiencing" tab in PageSpeed Insights.

Common mistakes that wreck your scores

- Loading uncompressed images as heavy JPEGs instead of WebP or AVIF. - Stacking third party scripts: live chat, advertising pixels, interactive maps, each one adding blocking JavaScript. - Forgetting the dimensions of images and ads, which causes layout shifts (CLS). - Loading heavy web fonts with no fallback strategy, delaying the display of your text. - Using a bloated generic theme (common on all in one CMS platforms) packed with features nobody needs.

Core Web Vitals checklist for a small business

- Images served as WebP/AVIF with explicit dimensions. - One single analytics tool and as few third party scripts as possible. - Web fonts with font-display: swap and preloading of the main font. - Fast hosting with a CDN enabled. - A monthly test on PageSpeed Insights, starting with the mobile report.

What slowness costs a small business

The Core Web Vitals are not only a search matter, they weigh directly on turnover. A tradesperson's brochure site that receives 1,500 visits a month with an LCP of 4.5 seconds loses a large share of its visitors before anything even appears. By bringing the LCP down to 1.8 seconds, it is realistic to recover 15 to 20% more visitors who stay, which is 225 to 300 extra people a month seeing the offer.

If 3% of those additional visitors request a quote, that is 7 to 9 qualified enquiries a month gained without a single extra euro of advertising. Over a year, the impact of one simple technical optimisation often exceeds the cost of a full redesign. This is why performance should be seen as a commercial investment, not as a developer detail.

Best practice in 2026

In 2026, INP is fully established as the main responsiveness metric, which penalises sites overloaded with JavaScript. The trend is a return to technical restraint: fewer scripts, server side rendering, and interfaces that respond instantly. For a small business it means that a light, well built site now beats a "rich" but slow one, including for local search.

At ConvertiLab, every site we build is optimised to score in the green on the Core Web Vitals. Next.js, server side rendering and image optimisation let us reach PageSpeed scores of 95 to 100, a concrete advantage for tradespeople and freelancers who want to stay ahead of their competitors on Google.