A Node.js app can be slow under load if the event loop is blocked by heavy computations or synchronous operations. Use asynchronous functions and worker threads to offload tasks.

Node.js applications can become slow when the event loop is blocked, especially under heavy load. This typically happens when CPU-intensive tasks (like calculations, data processing, or synchronous file/database operations) run on the main thread. Since Node.js is single-threaded, blocking the event loop causes other incoming requests to be delayed. To improve performance, move CPU-bound tasks to worker threads or child processes using Node’s built-in worker_threads module. For I/O-bound tasks, make sure you’re using asynchronous functions (e.g., fs.promises instead of fs.readFileSync()). By offloading heavy tasks and keeping the event loop free, you ensure that your Node.js app can handle high concurrency without slowing down.