[philipp-spiess]

The batching effect of a debounced input is very important. Running an update for every keystroke is expensive and you only want to run it with the latest input anyway. This is why Concurrent React will make batching the default behavior.

Internally, React manages a list of updates. Whenever you trigger one, it will be added to the list but the next re-render will only start after the previous one has finished (this is of course a simplified explanation but you get the idea).

This batching gives us the best of both worlds: We start rendering as soon as we have data and don’t have “idle” time in which the UI is unresponsive (like a long debounce function) but we still batch all updates between the different renderings so that we can skip individual updates to save time.

[Tade0]

ELI5, what's the benefit here?

Because now I see mostly the cost of having pieces of the render function shoehorned between input events just so that they can be discarded, because once the render function finishes it's grossly outdated.

[philipp-spiess]

Your users still want feedback _as they type_. The idea of this example though is that you want to see an update as soon as possible. If you use blur, you're also deferring the updates until the timeout is over for the first time - The timeout can only be approximated and will ultimately be more keystrokes behind as if we start rendering ASAP.

There is also the other problem with debounce that was pointed out earlier: The main thread _will be blocked_ when a long running tasks is executed, no matter _when_. You mentioned that the block would happen _after_ the user type in so it would be less noticeable but this is only an approximation. If you have animations on your website that use rAF, the block will still be very visible. Also if the user continues to type.

That said, I know that this is a contrived example and it might not be used as-such in a real world application. I also think it's not comparable to your auto-saving input example that would maybe be better off with using a debounce call. I'm sure that better examples for this use case will follow.

Edit: I can recommend Dan's talk at JSConf Iceland last year. He speaks about debounce as well: https://reactjs.org/blog/2018/03/01/sneak-peek-beyond-react-...