You pointed a good example of where this wouldn't work. However, you can still make everything else asynchronously fast; and use a loader when it needs to really wait for 5 secs.

For instance, take gmail. Some part might hard to use that way.. for instance, chat communication where you somewhat need to receive the answer of the other person to show it. However, adding labels, deleting a message, etc.. that can all be made asynchronously.