The verdict in three sentences
A slow B2B site loses leads before it is even read: every extra second of load time cuts conversion by roughly 7%. In 2026 in London, a Core Web Vitals audit and optimization cost GBP 2,500 to 7,000 depending on complexity, to reach LCP <2.5s, INP <200ms and CLS <0.1. The typical gain: +8 to +15% conversion and a mobile ranking lift within 4 to 8 weeks.
The three metrics that matter (and their thresholds)
Google evaluates three signals measured on real users (CrUX). Being green on all three is the condition for good mobile ranking in 2026.
| Metric | What it measures | Green threshold | Red threshold |
|---|---|---|---|
| LCP | Speed of main content render | < 2.5s | > 4.0s |
| INP | Responsiveness to interactions | < 200ms | > 500ms |
| CLS | Visual stability (layout shifts) | < 0.1 | > 0.25 |
| TTFB | Server response time | < 0.8s | > 1.8s |
Before / after: what optimization changes
Gains come mainly from image optimization (WebP/AVIF), lazy loading, JavaScript reduction and a CDN. On mobile-heavy audiences, these gains are amplified.
| Item | Before | After | Action |
|---|---|---|---|
| Mobile LCP | 4.8s | 2.1s | AVIF images + CDN + preload |
| INP | 420ms | 150ms | JS reduction, code splitting |
| CLS | 0.31 | 0.05 | Image dimensions + stable fonts |
| Page weight | 4.2 MB | 1.3 MB | Compression + lazy loading |
| Mobile PageSpeed score | 42/100 | 91/100 | Overall optimization |
| Conversion rate | 1.8% | 2.1% | Speed effect |
Typical timeline: 2 to 4 weeks for a mid-sized corporate or B2B site.
Mini case study
James, e-marketing lead at a B2B services firm in London, has a site at 42/100 on mobile and 3,200 visits/month. Optimization budget: GBP 4,000. After 3 weeks: mobile score at 91/100, LCP from 4.8s to 2.1s, conversion from 1.8% to 2.1%. On 3,200 visits, that takes leads from 58 to 67 per month, i.e. +9 leads. At a GBP 120 lead value, that is GBP 1,080 of additional monthly pipeline: the investment pays back in under four months from conversion alone, before the organic ranking gain.
FAQ
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What does a Core Web Vitals optimization cost in London in 2026?
Between GBP 2,500 and 7,000 depending on site size and technical debt. A simple corporate site sits around GBP 2,500-4,000; a complex JavaScript-heavy site rises to GBP 5,000-7,000.
What traffic or conversion gain should we expect?
Budget +8 to +15% conversion from speed alone, plus a mobile ranking gain. The SEO effect materializes in 4 to 8 weeks as Google refreshes CrUX data.
INP replaced FID, what changes?
INP measures responsiveness across the whole session, not just the first interaction; it is stricter. The target remains < 200ms, reached by reducing and splitting JavaScript.
Does a fast site really improve Google ranking?
Core Web Vitals are a ranking factor, mainly as a tie-breaker at equivalent content quality. The most measurable direct gain remains conversion; SEO is complementary.
How long does optimization take?
Budget 2 to 4 weeks for a mid-sized site, including audit, fixes and re-test. Very heavy or bespoke sites can take 6 weeks.
Let's scope your project. Send us your URL and current PageSpeed score and we will define the audit, priority fixes and budget. Detailed quote within 48 h. WhatsApp +221 77 596 93 33.
Mohamed Bah
Fondateur, Kolonell
Passionate about digital and entrepreneurship in Africa, Mohamed has been helping Sénégalese businesses with their digital transformation since 2020. Founder of Kolonell, he believes every SME deserves a professional and accessible online présence.
