If your Node.js app freezes under heavy I/O, it’s likely due to blocking operations that prevent the event loop from processing other tasks. Node.js operates on a single-threaded event loop, so any blocking code, such as synchronous file or database operations, will cause the entire app to stall. To prevent this, ensure all I/O operations are asynchronous. For example, use `fs.promises` instead of `fs.readFileSync()` for file operations. For CPU-intensive tasks, you can offload work to worker threads or child processes to prevent blocking the event loop. Properly handling I/O ensures your app remains responsive even under heavy load.