Yeah, the overall performance of the board is *terrible* right now; I'm not entirely sure what is causing it. Server load times are very bad, but I don't see any "smoking guns" in terms of particular HTTP requests that are causing problems.
I don't think the CDN caching should be too big a deal though, and because of the perf issues I'd like to minimize load onto the server as much as possible. The CDN uses the cache directives that come from the server, and I've checked that the only places that are caching right now are the image folders and not the JS/CSS. (I think that the CSS would actually be fine to add an expires directive to as well because according to what I've read, if I update style vars in vBulletin's admin page, it will create an entirely new CSS file to deal with cache expiration. But since the dust hasn't settled, I don't want to do that quite yet.