Why middleware intercepts requests in NextJS - Performance Evidence

Performance: Why middleware intercepts requests

MEDIUM IMPACT

Middleware interception affects the time before the main page content starts loading by running code on requests early in the server process.

Running middleware to check authentication on every request

NextJS

import { NextResponse } from 'next/server';

export function middleware(request) {
  // minimal quick check
  if (!request.cookies.get('auth')) {
    return NextResponse.redirect('/login');
  }
  return NextResponse.next();
}

Minimal logic avoids blocking, quickly passes request to next step or redirects early.

📈 Performance Gainreduces blocking to under 5ms, improving LCP

Running middleware to check authentication on every request

NextJS

import { NextResponse } from 'next/server';

export function middleware(request) {
  // heavy synchronous logic
  for (let i = 0; i < 1000000; i++) {}
  return NextResponse.next();
}

Heavy synchronous work blocks the request pipeline, delaying response start and LCP.

📉 Performance Costblocks rendering for 50-100ms depending on device

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Heavy synchronous middleware logic	0 (server-side)	0	Blocks initial paint	[X] Bad
Minimal quick middleware checks	0 (server-side)	0	Non-blocking initial paint	[OK] Good

Rendering Pipeline

Middleware runs before the main rendering pipeline starts, intercepting requests to modify or redirect them early. This affects the server response time and delays when the browser can start rendering.

→Request Handling

→Server Response

→Initial HTML Delivery

⚠️ BottleneckMiddleware processing time delays server response start, impacting LCP.

Core Web Vital Affected

LCP

Middleware interception affects the time before the main page content starts loading by running code on requests early in the server process.

Optimization Tips

1Keep middleware logic minimal to avoid blocking server response.

2Avoid heavy synchronous work inside middleware to improve LCP.

3Use middleware mainly for quick checks like authentication or redirects.

Performance Quiz - 3 Questions

How does heavy synchronous logic in Next.js middleware affect page load?

A It has no effect on page load performance.

B It speeds up the page load by pre-processing data.

C It delays the server response, increasing Largest Contentful Paint time.

D It only affects client-side rendering speed.

DevTools: Performance

How to check: Record a performance profile while loading a page with middleware. Look for long tasks in the server-side timeline before the first contentful paint.

What to look for: Long blocking tasks before the first paint indicate slow middleware delaying LCP.