The xhr system is necessarily sequential, as far as I can tell.
Javascript rolls along and decides it needs some data, and asks for it by xhr, and stops, and waits, and really can't do anything else until that data is returned.
There isn't any point to putting that download into a background thread, you still have to wait for it, so no, threading won't speed up any xhr requests.
And this is almost always dynamic data so not cacheable.

I guess a test might be a site with lots of js, like nasa, without any caching, and with or without jsbg.
Even if it doesn't speed things up a lot I'm glad I did it, for several reasons.

Karl Dahlke