Could one solve this problem with URLs of this form?

http://199.27.135.101/ajax/libs/prototype/1.7.0.0/prototype....

CloudFlare doesn't seem to support direct access based on IP, but there's no reason it couldn't, right? Do browsers support this?

I wasn't aware that a single DNS lookup was that expensive. Aren't they frequently cached? And with more people using cdnjs, there's a better chance the DNS entry as well as the assets might already be cached, leading to an even greater performance gain.

When you say "all these misc CDNs" you're talking about just one, CDNjs, right?

I suppose the CDN could host other assets if it made sense. But I can't think of any other assets with the type of commonality that JavaScript libs have.

"... the user has a slow DNS which is a fairly common problem now days"

I would find this somewhat surprising given the increased use of hosted DNS providers for both authoritative and recursive. Can you point to any data that corroborates this (not saying it isn't true, just looking for hard data)?

The real benefit of using one repository across multiple sites is that it increases the likelihood the library is available from the browser's cache. DNS lookup and retrieval will need to happen the first time a visitor goes to a site with a CDNJS library. If that same visitor visits another site that uses the same library then the library can be fetched from the browser's cache without any DNS or download latency. In other words, the more sites that use CDNJS, the bigger the benefit to the whole CDNJS community.