Core Web Vitals: How We Doubled a Client's Conversions
In early 2025, an e-commerce client came to us with a specific problem: their physical shop in Nantes was doing well, but the online store was barely converting. Decent traffic. Conversion rate: 0.4%. Industry average is 2–3%.
After an audit, the diagnosis was clear: Core Web Vitals were catastrophic. Six weeks later, conversions had doubled. Here's how.
What Are Core Web Vitals?
Core Web Vitals are the three performance metrics Google uses to evaluate user experience. They've been a ranking factor since 2021.
LCP (Largest Contentful Paint) — main content load time. Target: under 2.5s. Client had: 6.2s.
INP (Interaction to Next Paint) — response time to user interactions. Target: under 200ms. Client had: 820ms.
CLS (Cumulative Layout Shift) — visual stability, elements "jumping" during load. Target: under 0.1. Client had: 0.38.
What We Actually Did
WordPress to Next.js migration. The site ran on WordPress + WooCommerce with 43 active plugins. We migrated to a Next.js static build. Load time dropped from 6.2s to 0.9s.
Image optimization. All product images were unresized JPEGs at 3–4 MB each. We implemented next/image with automatic WebP conversion and lazy loading. Result: -70% on page weight.
Removing render-blocking resources. Two Google Fonts, a live chat widget, a Facebook pixel — all loading synchronously. We deferred everything non-critical to after the first render.
Layout shift fixes. Images without defined dimensions caused visual jumps during load. We added width and height attributes to every image element.
Results After 6 Weeks
| Metric | Before | After |
|---|---|---|
| LCP | 6.2s | 0.9s |
| INP | 820ms | 140ms |
| CLS | 0.38 | 0.04 |
| Lighthouse Score (Mobile) | 31 | 94 |
| Conversion Rate | 0.4% | 0.9% |
| Monthly Online Revenue | baseline | +127% |
Why Performance Is a Conversion Variable
The correlation between load speed and conversion is well-documented. Google found that each additional second of delay reduces conversions by 7%. Amazon estimated a 1% revenue loss per second of additional delay.
A fast website isn't just "pleasant" — it's a direct business variable that compounds over time.
How to Check Your Core Web Vitals
- Go to PageSpeed Insights and enter your URL
- Check the Mobile score — that's what Google prioritizes
- LCP > 4s or score < 50? That's an urgent priority.
Want us to audit your site? Nervure offers free performance audits for businesses in Loire-Atlantique — results in 48 hours.
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