The event loop of a Node.js process is a single thread that polls for incoming connections and executes all application code. When a Node.js request makes a call to an external database, remote service or the filesystem, the event loop automatically directs the application's control flow to some other task, including other connections or callbacks.

CPU-intensive operations block the event loop, preventing it from handling incoming requests or finishing existing requests. A CPU-intensive operation in one business transaction may cause slowness in other business transactions.