To write a modern responsive UI application that always runs smoothly, it is absolutely necessary to never block the main thread for more than a few milliseconds. A smooth running app is updating its interface 60 times a second. This means that if you are doing something that blocks the UI thread for more than 16 ms, the FPS will drop.
You can design your internal APIs in an application with this in mind. Whenever you write a function that performs I/O or something else that might take more than a few milliseconds, it needs to be implemented as an asynchronous function. This pattern has become very common in iOS and Windows where, for example, all network APIs have become asynchronous.
