Building custom middleware in Node.js - Performance & Optimization

Performance: Building custom middleware

MEDIUM IMPACT

This affects server response time and how quickly the server processes requests before sending responses.

Processing HTTP requests with custom middleware in Node.js

Node.js

app.use(async (req, res, next) => {
  await someAsyncOperation();
  next();
});

Using asynchronous operations prevents blocking the event loop, allowing other requests to be handled concurrently.

📈 Performance GainNon-blocking middleware improves throughput and reduces response delays.

Processing HTTP requests with custom middleware in Node.js

Node.js

app.use((req, res, next) => {
  // heavy synchronous computation
  for (let i = 0; i < 1e8; i++) {}
  next();
});

Blocking the event loop with heavy synchronous code delays all requests, increasing response time.

📉 Performance CostBlocks event loop for hundreds of milliseconds, increasing server response time and INP.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Blocking synchronous middleware	0 (server-side)	0 (server-side)	0 (server-side)	[X] Bad
Asynchronous non-blocking middleware	0 (server-side)	0 (server-side)	0 (server-side)	[OK] Good

Rendering Pipeline

Middleware runs on the server before sending responses, affecting how fast the server can start sending data to the browser.

→Request Handling

→Response Preparation

⚠️ BottleneckBlocking synchronous code in middleware delays response start time.

Core Web Vital Affected

INP

This affects server response time and how quickly the server processes requests before sending responses.

Optimization Tips

1Avoid heavy synchronous code in middleware to prevent blocking the event loop.

2Use asynchronous operations and call next() only after completion.

3Keep middleware logic simple and fast to improve server response time.

Performance Quiz - 3 Questions

What is the main performance risk of using heavy synchronous code in custom middleware?

A It increases the size of the JavaScript bundle sent to the client.

B It blocks the event loop, delaying all incoming requests.

C It causes layout shifts in the browser.

D It improves server throughput by batching requests.

DevTools: Network and Performance panels

How to check: Use Network panel to measure server response time; use Performance panel to check request blocking time.

What to look for: Look for long server response times or blocking periods indicating slow middleware processing.