Should I link to the full jQuery UI -or- provide a slimmed-down customized copy?
There is a very significant size difference between the full jQuery-UI from a CDN like Google vs. providing a customized minimal version. Additionally, one can combine jQuery, jQuery UI, plug-ins, and local code into one single minified JavaScript download.
On the other hand, the chances of a user having a customized copy cached are nil unless they’ve visited the site previously.
Are there any jQuery CDN “penetration” stats or studies that lend insight into which might be better?
I usually take a look at the components I would be using from jQuery UI and only if I am using 50% or more of the components in it I would use the full UI from the CDN.
Here is some great reasoning and (a bit outdated) stats from http://zoompf.com/blog/2010/01/should-you-use-javascript-library-cdns
The issue obviously is that not all sites do it, and not all use the same CDN which keeps lowering the probability for a hit. On top of that then there’s versioning, if you need an older version then that’ll keep lowering the probability.
I dfntly use it for jQuery itself since the probability for a hit seems higher and also the other benefits that come with it defeat the overhead of DNS lookups.