but there isn't 1 CDN, or 1 version... if you need two libraries, but the canonical CDN for jquery is on one, and your required extension is on another... that's two DNS lookups, connections, request cycles, etc.
So you use the one that has both, but one is not canonical, which means more cache misses. That doesn't even count the fact that there are different versions of each library, each with it's own uses, and distribution, and the common CDN approach becomes far less valuable.
In the end, you're better off compositing micro-frameworks and building yourself. Though this takes effort... React + Redux with max compression in a simple webpack project seems to take about 65K for me, before actually adding much to the project. Which isn't bad at all... if I can keep the rest of the project under 250K, that's less than the CSS + webfonts. It's still half a mb though... just the same, it's way better than a lot of sites manage, even with CDNs
that's 2 dns lookups on any user that hasn't already done that somewhere in the past and had it cached.
The question then is how likely are they to have done that in regards to your particular cdn and version of the library.
I agree that a lot of possible cdns, versions and so forth decreases the value of the common CDN approach, but there are at least some libraries that have a canonical CDN (JQuery for example) and not using that is essentially being the selfish player in a games theory style game.
Since I don't know of any long running tracking of CDN usage that allows you to predict how many people who visit your site are likely to have a popular library in their cache it's really difficult to talk about it meaningfully (I know there are one-off evaluations done in one point in time but that's not really helpful).
Anyway it's my belief that widespread refusal to use CDN versions of popular libraries is of course beneficial in the short run for the individual site but detrimental in the long run for a large number of sites.
So you use the one that has both, but one is not canonical, which means more cache misses. That doesn't even count the fact that there are different versions of each library, each with it's own uses, and distribution, and the common CDN approach becomes far less valuable.
In the end, you're better off compositing micro-frameworks and building yourself. Though this takes effort... React + Redux with max compression in a simple webpack project seems to take about 65K for me, before actually adding much to the project. Which isn't bad at all... if I can keep the rest of the project under 250K, that's less than the CSS + webfonts. It's still half a mb though... just the same, it's way better than a lot of sites manage, even with CDNs