I think you’re overestimating the impact of this. Most web site content these days are served from the web site owner’s own domain.
It’s only if a.com and b.com have (for example) the exact same image URL (c.com/img123.jpg) embedded, and you visit both sites, that this cache partitioning will make a difference.
In essence, there’s very little legitimate Internet traffic that would be effected by this change, but lots and lots of creepy spyware behaviour will be prevented.
What about JS libraries or CSS hosted by a CDN? I'm thinking jQuery, Bootstrap, etc etc. I learned that using a common CDN was the way to go because the content would likely already be in the user's cache and often not need to be loaded.
This was discussed when Chrome made this change. It makes almost no difference because to get any saving you have to have lots of websites that use the same CDN and the same version of jQuery etc. Unlikely enough to not matter.
Indeed, and the savings are fairly small even in the best case, jQuery is 28kB gzipped, a drop in the ocean of the multi-megabyte payload of most big sites these days.
CDNs are/were not only or even primarily used for caching but to minimize bulk traffic to your site and prevent hitting max concurrent per-domain HTTP request limits and HTTP/1.1 head-of-line blocking.
I see what you're saying. But, for example, all of the new DNS queries for things like jQuery and Google Analytics surely add up to something noticeable.
You’d be better off installing a caching proxy, so that all connections from all of your devices share one cache, rather than only altering settings in one browser.
If you’re a Mac user with more than one of any kind of Apple device on your network (like, two Macs), you can install their Server app on any macOS and enable software update caching as well.
Is there a way to disable it? Or should I better think about installing a caching proxy to avoid the redundant traffic?