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Case Study: How We Improved Core Web Vitals by 60% in Two Weeks

2026-08-278 min readEduard Tymchenko
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Core Web Vitals improvement case study showing LCP INP and CLS optimization results on Next.js site
Table of Contents

The Before

A Next.js ecommerce site, 12K SKUs, $2M monthly revenue. Their organic traffic had been declining for six months. Google Search Console showed the problem: 68% of URLs in the "poor" LCP bucket.

After: 92% "good" across all three metrics. Here's exactly what we changed.

LCP: 4.2s → 1.1s

The hero image on every product page was loading at full resolution — 4000x4000px, 2.3MB. The tagline said "optimized" but they were using a generic CDN without WebP conversion. For a complete breakdown of diagnosing and fixing LCP, see our LCP deep dive guide.

What we did:

  • Converted all hero images to WebP with 85% quality
  • Added fetchpriority="high" to the LCP image
  • Implemented responsive image sizes via Next.js Image component with sizes attribute
  • Moved the third-party review widget below the fold (it was loading 180KB of JS before the hero)

The biggest win was the review widget. Removing one