I think the concept itself is ridiculous. It's 2013 guys, UI response should be instant. If it's not, you need to rethink what you're doing and ponder your life in general.
You're right that we should all strive to build UI that responds instantly. Unfortunately sometimes, for reasons out of a designers control there might be a legitimate reason for a site to reach that level - for that, adding a subtle progress bar does help.
Think of it as a local optimisation before you reach a global one of instant UI.
Are you implying that the author would have better spent his time redesigning HTTP rather than making this library? Because to me, that is even more ridiculous.
But it is not instant. This is used when you are making an external HTTP request over the Internet, to a server that may or may not respond instantly (if at all), over a link that may be saturated at any point in time.
Think of it as a local optimisation before you reach a global one of instant UI.