Why optimization matters for performance in NextJS - Performance Evidence

Performance: Why optimization matters for performance

CRITICAL IMPACT

Optimization affects how fast the page loads and how smoothly it responds to user actions.

Loading a Next.js page with many unoptimized images and scripts

NextJS

import Image from 'next/image';

export default function Page() {
  return (
    <>
      <Image src="/large-image.jpg" alt="Large" width={600} height={400} priority />
      <script async src="/heavy-script.js"></script>
      <div>Content</div>
    </>
  )
}

Using Next.js Image optimizes loading and async script loading avoids blocking rendering.

📈 Performance GainReduces LCP by 40%, avoids blocking main thread, improves INP and CLS

Loading a Next.js page with many unoptimized images and scripts

NextJS

export default function Page() {
  return (
    <>
      <img src="/large-image.jpg" alt="Large" />
      <script src="/heavy-script.js"></script>
      <div>Content</div>
    </>
  )
}

Loading large images and blocking scripts without optimization delays page rendering and interaction.

📉 Performance CostBlocks rendering for 500ms+, triggers multiple reflows, increases LCP and INP

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Unoptimized images and blocking scripts	High (many nodes and large assets)	Multiple reflows due to delayed layout	High paint cost from large images	[X] Bad
Optimized images with Next.js Image and async scripts	Moderate (optimized nodes)	Single reflow after fast layout	Low paint cost with compressed images	[OK] Good

Rendering Pipeline

Unoptimized assets block the critical rendering path causing delays in style calculation, layout, and paint. Optimized assets load faster and allow the browser to paint content sooner.

→Critical Rendering Path

→Style Calculation

→Layout

→Paint

⚠️ BottleneckBlocking scripts and large unoptimized images delay the Critical Rendering Path

Core Web Vital Affected

LCP, INP, CLS

Optimization affects how fast the page loads and how smoothly it responds to user actions.

Optimization Tips

1Always optimize images using Next.js Image component for faster loading.

2Load scripts asynchronously to avoid blocking the main thread.

3Split code and lazy load components to reduce initial bundle size.

Performance Quiz - 3 Questions

What is the main reason to optimize images in a Next.js app?

A To increase the number of DOM nodes

B To make images look sharper on all screens

C To reduce page load time and improve Largest Contentful Paint

D To add more JavaScript to the bundle

DevTools: Performance

How to check: Record a page load and interaction in the Performance panel, then analyze the Main thread activity and Largest Contentful Paint event timing.

What to look for: Look for long tasks blocking the main thread, delayed LCP marker, and multiple layout shifts indicating poor optimization.