CDN is supposed to be fast, right? CDN is supposed to off-load your servers, right? CDN is supposed to be geographically boosted for the visitor, right? But when you have 3-6 *different* CDN sources in your various scripts/include list, and they’re all HTTPS (as they should be), the browser will perform a TLS/SSL handshake 3-6 times when it loads your website. This is by no means fast. In fact, if you’re tweaking your website for below-one-second-load-times, you may be barking up the wrong tree.
If you instead place those resources locally, on your own server secured with HTTPS, you get one session handshake. And since most “clever” sites tell the browser(s) to cache static resources like .js, images, and so on, I’m not so sure I think CDN is the way to go for many websites if you use different CDN resources on the same website, that is.
But hey, what do I know, I don’t run the CDN companies 🙂