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.