Why Node.js APIs Become Slow Over Time (And How to Fix It)

Many developers start with a fast Node.js API, but after weeks or months in production, performance slowly degrades. Response times increase, memory usage grows, and the server feels unstable.

This is a common problem — and in most cases, it has very specific causes.

In this article, we’ll explore why Node.js APIs become slow over time and how to fix the most frequent issues.

How Node.js Handles Requests

Node.js uses a single-threaded event loop. This allows it to handle many concurrent requests efficiently — as long as the event loop is not blocked.

When the event loop is blocked or overloaded, performance drops quickly.

Problem 1: Blocking the Event Loop

One of the most common causes of slow Node.js APIs is blocking code.

Examples include:

• Heavy CPU calculations
• Synchronous file system operations
• Large JSON parsing

Bad Example (Blocking Code)

app.get('/data', (req, res) => {
  const result = heavyCalculation(); // blocks event loop
  res.json(result);
});

Solution

• Move CPU-heavy tasks to worker threads
• Use async APIs
• Offload work to background services

Problem 2: Memory Leaks

Node.js does not magically clean everything. Memory leaks cause APIs to slow down over time.

Common causes:

• Uncleared intervals or timeouts
• Global variables holding references
• Caching without limits

Bad Example (Unbounded Cache)

const cache = {};

app.get('/user/:id', (req, res) => {
  cache[req.params.id] = getUser(req.params.id);
  res.json(cache[req.params.id]);
});

Solution

• Use LRU caches
• Set expiration policies
• Monitor heap usage

Problem 3: Too Many Open Connections

Database and external API connections are expensive.

Symptoms include:

• Slow responses
• Connection timeouts
• High memory usage

Bad Example (No Connection Pooling)

app.get('/orders', async (req, res) => {
  const db = await connectToDatabase();
  const orders = await db.query('SELECT * FROM orders');
  res.json(orders);
});

Solution

• Reuse database connections
• Configure connection pools
• Close unused connections

Problem 4: Uncontrolled Logging

Logging is useful — but excessive logging kills performance.

Writing logs synchronously or logging every request can slow APIs dramatically.

Solution

• Use async logging libraries
• Reduce log level in production
• Avoid logging inside hot paths

Problem 5: No Monitoring or Metrics

Many slow Node.js APIs fail because developers don’t know what’s happening.

Without metrics, performance problems go unnoticed until users complain.

What to Monitor

• Event loop delay
• Memory usage
• CPU load
• Response times

How to Keep Node.js APIs Fast

• Avoid blocking the event loop
• Manage memory carefully
• Use connection pooling
• Monitor performance continuously

Conclusion

Node.js APIs don’t become slow by accident. In most cases, performance issues are caused by a small set of common mistakes.

By understanding how Node.js works and fixing these issues early, you can keep your APIs fast and reliable even under heavy load.