> I don’t think it ever even improved page load speeds, because it introduces another dns request, another tls handshake, and several network round trips just to what?

I think the original use case, was when every site on the internet was using jquery, and on a js based site this blocked display (this was also pre fancy things like HTTP/2 and TLS 0-RTT). Before cache partitioning you could reuse jquery js requested from a totally different site currently in cache as long as the js file had same url, which almost all clients already had since jquery was so popular.

So it made sense at one point but that was long ago and the world is different now.