---
title: "Core Web Vitals on Next.js: from 4.5s LCP to 0.9s in 3 weeks"
description: "A real audit walkthrough: image priority, RSC boundaries, edge caching, font loading · and how these shifts feed back into Google ranking."
date: 2026-02-14
updated: 2026-02-14
author: "Dezső Mező"
tags: "Web, Performance, Next.js, SEO"
slug: core-web-vitals-nextjs-case-study
canonical: https://dfieldsolutions.com/blog/core-web-vitals-nextjs-case-study
---

# Core Web Vitals on Next.js: from 4.5s LCP to 0.9s in 3 weeks

A real case: image priority, RSC boundaries, edge caching, font loading · 4 seconds of LCP saved in three weeks.
[Core Web Vitals](https://web.dev/vitals/) is a Google ranking factor, and the user feels the difference. Here's a real audit of a [Next.js](https://nextjs.org/)-based B2B SaaS marketing site · anonymised, but the numbers are exact. We run this audit under our [Web service](/services/web).

## Starting point

- LCP: 4.5s (red)
- CLS: 0.21 (red)
- INP: 420ms (amber)
- TTFB: 1.1s (amber)

## 1. Hero image + font: the biggest visible win

The hero image wasn't priority-loaded, and the custom font with font-display: auto held back the first paint. Two tiny changes (-1.4s LCP):

```tsx
// Before
<Image src={heroImg} alt="" />

// After
<Image
  src={heroImg}
  alt="Hero illustration"
  priority
  fetchPriority="high"
  sizes="(min-width: 1024px) 600px, 100vw"
/>
```

## 2. RSC boundaries: carve out the slow parts

On the marketing page, nothing needed to be interactive above the fold. Switching to RSC reduced hydration payload from 110kb to 22kb; INP dropped from 420ms to 180ms.

## 3. Edge + ISR

Blog pages got 300s ISR + edge runtime. TTFB fell from 1.1s to 90ms. Guests from Singapore now hit the home page in sub-second.

## 4. CLS: current 0.00

- Reserve height for ads / embeds · no reflow.
- font-size-adjust for fallback · no text jump.
- Aspect-ratio on every image.

## Result, after 3 weeks

- LCP: 0.9s (green)
- CLS: 0.01 (green)
- INP: 120ms (green)
- TTFB: 90ms (green)
- Organic traffic +34% in three months

## Takeaway

You don't need to rebuild the whole app. 3-5 targeted changes on 20% of the user journey is enough for CWV green. We can run the audit on you · 1-2 weeks for the audit + the fix PRs.

---

Source: https://dfieldsolutions.com/blog/core-web-vitals-nextjs-case-study
Author: Dezső Mező · Founder, DField Solutions
Site: https://dfieldsolutions.com