I don't understand the question, it's either cached or not.

If it's not cached, then it has to be fetched synchronously for anything depending on its value to work - so it's slow.

If it is cached, then the cached value is known.

> Why not cache it for the user, but go ahead and retrieve it in the background?

If the JS triggers another download, and the browser requests the second resource before the initial JS is done downloading...

The point is that other scripts on the page can find out that id=0x1234567, and use that value for tracking requests. Since you've cached it, they can track you across sessions.

The idea is that if you visit two sites, and on both sites you use the same token, the ad-provider (or whoever) can associate you across the two origins because of the cached token.