Creating middleware in NestJS - Performance Optimization Steps

Performance: Creating middleware

MEDIUM IMPACT

Middleware affects request processing speed and can impact server response time and client perceived latency.

Adding logging to requests

NestJS

import { Injectable, NestMiddleware } from '@nestjs/common';
import { Request, Response, NextFunction } from 'express';

@Injectable()
export class LoggingMiddleware implements NestMiddleware {
  use(req: Request, res: Response, next: NextFunction) {
    setImmediate(() => {
      console.log(`Request to ${req.url}`);
    });
    next();
  }
}

Defers logging asynchronously, allowing the request to continue without blocking the event loop.

📈 Performance GainNon-blocking middleware reduces INP and improves server responsiveness.

Adding logging to requests

NestJS

import { Injectable, NestMiddleware } from '@nestjs/common';
import { Request, Response, NextFunction } from 'express';

@Injectable()
export class LoggingMiddleware implements NestMiddleware {
  use(req: Request, res: Response, next: NextFunction) {
    // Synchronous heavy computation
    for (let i = 0; i < 1e7; i++) {}
    console.log(`Request to ${req.url}`);
    next();
  }
}

Heavy synchronous work blocks the event loop, delaying all requests and responses.

📉 Performance CostBlocks event loop for 50-100ms per request, increasing INP and overall latency.

Performance Comparison

Pattern	Event Loop Blocking	Request Latency	Concurrency Impact	Verdict
Synchronous heavy work in middleware	High (blocks event loop)	High latency per request	Low concurrency	[X] Bad
Asynchronous non-blocking middleware	None (event loop free)	Low latency	High concurrency	[OK] Good

Rendering Pipeline

Middleware runs during the server request processing pipeline before the final response is sent. It affects how fast the server can respond and how quickly the client receives data.

→Request Handling

→Event Loop

→Response Generation

⚠️ BottleneckBlocking synchronous code in middleware stalls the event loop, delaying all requests.

Core Web Vital Affected

INP

Middleware affects request processing speed and can impact server response time and client perceived latency.

Optimization Tips

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

2Use asynchronous operations in middleware to keep the server responsive.

3Keep middleware logic minimal and non-blocking to improve request handling speed.

Performance Quiz - 3 Questions

What is the main performance risk of synchronous heavy computation inside NestJS middleware?

A It increases CSS paint time in the browser.

B It blocks the event loop, delaying all requests.

C It causes layout shifts on the client side.

D It reduces the bundle size.

DevTools: Performance

How to check: Record a server profile or use Node.js profiling tools to check event loop delays during request handling.

What to look for: Look for long blocking tasks or event loop stalls indicating synchronous blocking in middleware.