Why middleware processes requests before handlers in NestJS - Performance Evidence

Performance: Why middleware processes requests before handlers

MEDIUM IMPACT

This affects the time it takes for the server to start processing a request and the overall responsiveness of the application.

Preprocessing requests before reaching route handlers

NestJS

app.use(async (req, res, next) => {
  // lightweight async preprocessing
  await Promise.resolve();
  next();
});
app.get('/data', (req, res) => {
  res.send('Data');
});

Async and minimal middleware avoids blocking, allowing faster handler execution.

📈 Performance GainReduces blocking time to near 0ms, improving INP and responsiveness.

Preprocessing requests before reaching route handlers

NestJS

app.use((req, res, next) => {
  // heavy synchronous processing
  for (let i = 0; i < 1e7; i++) {}
  next();
});
app.get('/data', (req, res) => {
  res.send('Data');
});

Heavy synchronous work in middleware blocks the event loop, delaying all request handling.

📉 Performance CostBlocks request processing for 100+ ms, increasing INP and slowing response.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Heavy synchronous middleware	N/A (server-side)	N/A	N/A	[X] Bad
Lightweight async middleware	N/A (server-side)	N/A	N/A	[OK] Good

Rendering Pipeline

Middleware runs early in the request lifecycle, before route handlers, allowing preprocessing like authentication or logging. This stage affects how quickly the server can start generating a response.

→Request Parsing

→Middleware Execution

→Route Handler Execution

⚠️ BottleneckMiddleware Execution if it contains heavy synchronous code

Core Web Vital Affected

INP

This affects the time it takes for the server to start processing a request and the overall responsiveness of the application.

Optimization Tips

1Middleware runs before handlers to preprocess requests.

2Avoid heavy synchronous work in middleware to prevent blocking.

3Use async operations in middleware to keep the event loop free.

Performance Quiz - 3 Questions

Why does middleware run before route handlers in NestJS?

A To delay the response intentionally

B To preprocess requests and possibly modify them before handlers run

C To run after the response is sent

D To handle errors only

DevTools: Performance

How to check: Record a server-side profile or use Node.js profiling tools to measure middleware execution time before handlers.

What to look for: Look for long blocking tasks in middleware that delay route handler start times.