Website speed has a direct effect on sales, at a rate of -7% conversions for every extra second of load time. On a site with 1,000 visits a month and an average basket of 200€, going from 3 seconds to 5 seconds of load time costs 14% of revenue, without you ever noticing.
In 2026, Google builds the Core Web Vitals into its ranking algorithm. A slow site loses positions and traffic at the same time as it loses conversions. Across the 150+ ConvertiLab sites, the average mobile PageSpeed score at handover is 88/100.
What are the Core Web Vitals and why does Google use them to rank sites?
The Core Web Vitals are 3 metrics Google has used since 2021 to judge the user experience of a website and feed that into its rankings. They measure load speed (LCP), visual stability (CLS) and responsiveness to interaction (INP). A site with good Core Web Vitals scores will rank above a competitor with equivalent content.
LCP (Largest Contentful Paint)
LCP measures how long it takes to display the largest visible element on the page, usually the main image or the main block of text. Google treats an LCP under 2.5 seconds as good, between 2.5 and 4 seconds as needing work, and beyond 4 seconds as poor.
INP (Interaction to Next Paint)
INP measures how responsive your site is when someone interacts with it (a click, a tap, typing). An INP under 200 ms is good. This is the metric that replaced FID (First Input Delay) in 2024.
CLS (Cumulative Layout Shift)
CLS measures the visual stability of the page. If elements move about while the page loads, say a button that jumps when an advert appears, the CLS is high. A score under 0.1 is good.
To find out your current scores, use the free Speed Check tool.
| Metric | Good | Needs work | Poor | What it measures |
|---|---|---|---|---|
| LCP | <2.5s | 2.5s–4s | >4s | How fast the main content appears |
| CLS | <0.1 | 0.1–0.25 | >0.25 | Visual stability (elements that shift) |
| INP | <200ms | 200ms–500ms | >500ms | Responsiveness to clicks and interactions |
What does a slow site actually do to your conversions and your revenue?
A slow site hits conversions and revenue through 3 mechanisms: visitors leave before they have seen the offer (-7% conversions per second), Google downgrades your ranking (less traffic), and your brand image suffers (it looks unprofessional). On a shop turning over 5,000€/month, going from 2s to 5s of load time can cost 700€ a month in invisible revenue.
Key figures on the impact of speed:
A slow site also sends a signal that you are not professional: if your site takes 5 seconds to load, a competitor loading in 1.5s will capture your visitors before they have seen what you offer.
Another angle that often gets ignored: on mobile over an average 4G connection, not fibre, the differences in speed are amplified. A site that loads in 2s on fibre can take 6s on 4G out and about, which is exactly where your customers are looking for you on their phone.
What are the most common causes of a slow website?
The 5 most common causes of a slow website are: unoptimised images (heavy JPEG instead of light WebP), too much JavaScript (plugins, trackers, widgets), undersized hosting (shared hosting at 3€/month), no caching, and unoptimised code (unused CSS, slow database queries). These 5 causes account for 80% of the slowness we have identified across 150+ ConvertiLab audits.
| Cause | How often | Score impact | Fix |
|---|---|---|---|
| Unoptimised images | 85% of sites | LCP +2-4s | WebP plus lazy loading |
| Too much JavaScript | 70% of sites | INP degraded | Audit and remove plugins |
| Shared hosting | 60% of sites | TTFB >600ms | Move to Vercel/CDN |
| No caching | 50% of sites | Every metric | Cache-Control headers |
| Unoptimised code | 40% of sites | Variable | Technical audit |
How do you actually make your website faster?
To make your website faster in practice, the 4 most effective actions are: convert every image to WebP with lazy loading (average gain of 15-20 PageSpeed points), move to CDN hosting (Vercel is free for Next.js sites), remove non essential third party scripts, and turn on caching. Those 4 actions are enough to take a site from 50/100 to 80+/100 in 70% of cases.
Have a look at our website design services, delivered with a mobile PageSpeed score of 85+/100.
Frequently asked questions about website speed
How can I measure my website speed for free?
Use 3 complementary tools: Google PageSpeed Insights (results against the real Core Web Vitals), the ConvertiLab Speed Check tool (a combined analysis with prioritised recommendations) and Chrome DevTools → Lighthouse (a local audit with no cache). Always test mobile and desktop separately, because the scores can differ by 30 to 50 points between the two.
Is speed enough on its own to rank well on Google?
No, speed is one factor among many (content, backlinks, domain authority, search intent). But when content and authority are equal, performance makes the difference. And a fast site indirectly improves other SEO signals: a lower bounce rate, longer sessions and more pages viewed, all signals that Google reads favourably.
How long does it take to speed up an existing site?
The quick wins (WebP images, removing unnecessary scripts, turning on caching) take 1 to 2 days on an existing site and can gain 20 to 40 PageSpeed points. A deeper technical rebuild (moving from WordPress to Next.js) takes 3 to 6 weeks but usually lifts the score from 40-55/100 to 85-95/100. A real case: a firm of architects in Bordeaux had a site with 35 JPEG photos of 4-8 MB each. PageSpeed score: 18/100, load time: 11 seconds. After converting the images to WebP and turning on lazy loading, the score rose to 74/100 in 4 hours of work, without touching the code or the design. The bounce rate fell from 76% to 52% in 2 weeks.
Does a good PageSpeed score guarantee a site that brings in customers?
No. PageSpeed measures technical performance, not the quality of your content, your copywriting or your offer. An extremely fast site with a vague message and no visible call to action will not convert. Speed is necessary but not sufficient. Think of it as a prerequisite: without speed you have a technical problem blocking everything else. With speed, you can work on what actually brings in customers, namely your value proposition, your social proof and the user experience. The right sequence: fix the speed problems first, because they block everything else, then optimise the content and the design. That is the method we follow at ConvertiLab on every redesign: performance first, conversion second. !PageSpeed score and technical audit, prioritised recommendations and conversion gains
