What happens if you perform a synchronous operation inside the main thread?

If you perform a synchronous operation on the main (UI) thread in iOS, the user interface will become unresponsive to user actions until the operation completes. This happens because the main thread is responsible for event handling and UI updates, and blocking this thread causes the interface to "freeze."

For example, if data loading from the network or heavy processing is done synchronously on the main thread, the app may appear frozen, which worsens user experience.

To avoid this, long-running operations should be performed on a background thread using GCD (Grand Central Dispatch) or OperationQueue, and UI updates should be done back on the main thread.