> Such a redirect isn't slow? It's near instant if done right. Here's twitter doing it: http://twitter.com/bjorntipling

On slower devices (such as mobile phones) redirecting can be more noticeable. You're right though, in general it's not really a problem, but might be more so than a larger-than-normal JS file.

> Large JS files only need to be downloaded once and are cached.

Yeah, I was trying to say that. Larger JS files aren't a problem at all; load them once and they're cached.