The popular stuff should be just as performant as if it were packaged with the browser - most sites are pulling jquery from one of the big CDNs and caching it for as long as possible. You aren't downloading jquery for every site that needs it, you're downloading it once for the thousands of sites that use the same cdn. Packaging it into the browser is just going to introduce a whole bunch of complexity around versioning and updates and standardization.
You aren't downloading jquery for every site that needs it, so long as they are using the same CDN and the same version of jquery. I wonder what the hit rate really is...
I think people seriously overestimate how useful CDNs are for static assets. Even if the page you visit happens to call for the same version of a resource you might have cached, there is no guarantee it is requested from the same CDN. And all these hostname lookups slow things down too. I frequently see a site request assets from multiple different CDNs when those assets are all available from a single CDN. The designer clearly copied a snippet of HTML without regard for the extra lookups when they could have found all the necessary assets on a single CDN.
I tried finding reliable data for cache hit/miss rates for popular CDNs but was unsuccessful. Is there a site out there tracking this that designers could refer to? It would be good to know if simply concatenating all your CSS (for example, or JS) into one file and serving it from Cloudfront would beat scattering it across several providers and hoping the majority of visitors had cached copies. I suspect the single file would perform better but I have no reliable data to support this.
