The verdict in three sentences
Degraded Core Web Vitals penalize both your Google ranking and your conversion rate. A performance audit costs 1,200-2,500 EUR and the optimization work 3,000-7,000 EUR, over a 2-4 week timeline. The goal: get under LCP 2.5s and INP 200ms, with a measured conversion gain of +8-15%.
Understanding the three metrics and their thresholds
Google evaluates three signals: LCP (time to render the largest element), INP (interaction responsiveness, which replaced FID in 2024) and CLS (visual stability). Each metric has a "good" threshold, to be met at the 75th percentile of real visits.
| Metric | Good | Needs work | Poor | Main lever |
|---|---|---|---|---|
| LCP | < 2.5s | 2.5 - 4s | > 4s | Images, CDN, hosting |
| INP | < 200ms | 200 - 500ms | > 500ms | JavaScript, third parties |
| CLS | < 0.1 | 0.1 - 0.25 | > 0.25 | Media dimensions, fonts |
| TTFB | < 800ms | 800 - 1800ms | > 1800ms | Server, cache |
Before / after by lever
A performance job proceeds by levers prioritized on their impact/cost ratio. Here are typical gains seen on a WordPress marketing site in 2026.
| Lever | Cost (EUR) | Before | After | Impact |
|---|---|---|---|---|
| WebP/AVIF image compression | 600 - 1,200 | LCP 4.8s | LCP 3.1s | high |
| Lazy-load + critical preload | 500 - 900 | LCP 3.1s | LCP 2.3s | high |
| JavaScript / third-party reduction | 1,000 - 2,500 | INP 420ms | INP 160ms | high |
| Media space reservation | 400 - 800 | CLS 0.28 | CLS 0.04 | medium |
| CDN + server cache | 800 - 1,600 | TTFB 1,400ms | TTFB 380ms | medium |
The 2026 figures are an order of magnitude; a well-built custom site usually needs fewer fixes than a WordPress overloaded with plugins.
Mini case study
Sarah is head of marketing at a B2B services SME in London. Her site converts at 1.8% with a 4.6s LCP. She invests 1,800 EUR in the audit and 5,200 EUR in the work, totaling 7,000 EUR. After optimization, LCP drops to 2.2s and conversion rises to 2.1%, a +16% gain. On 9,000 visits/month and a 3,200 EUR average deal, that is 5 extra leads/month; the investment pays back in under 2 months.
FAQ
Need a professional website?
Kolonell builds websites that attract clients, optimized for the Sénégalese market. Free quote in 2 minutes.
Does a fast site really improve SEO?
Yes, but indirectly: Core Web Vitals are one ranking factor among many, and above all a conversion factor. With equal content, the faster site gains positions and converts better.
What is the difference between lab and field data?
Lighthouse measures in the lab; Google ranks on field data (CrUX) from real visitors. You optimize for the field, at the 75th percentile.
Why is my INP poor when the site feels fast?
INP measures responsiveness to clicks and input, often dragged down by third-party scripts (chat, analytics, ads). Reducing or deferring those scripts is the main lever.
How long do the gains last?
Gains last as long as you avoid adding heavy new scripts. An annual check-up audit (600-1,000 EUR) is enough to keep the level.
Should I rebuild the site or optimize it?
If the technical base is sound, optimization is enough. If the site has accumulated plugins and technical debt, a light rebuild is often cheaper long term.
Let's scope your project. Send us the URL to audit and your conversion goals, and we'll quote an impact-prioritized audit and work plan. 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.
