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? Save a few kb on your js bundle size? That’s not a good deal! Just bundle small polyfills directly. At these sizes, network latency dominates download time for almost all users.
> 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.
I believe you could download from multiple domains at the same time, before HTTP/2 became more common, so even with the latency you'd still be ahead while your other resources were downloading. Then it became more difficult when you had things like plugins that depended on order of download.
You can download from multiple domains at once. But think about the order here:
1. The initial page load happens, which requires a DNS request, TLS handshake and finally HTML is downloaded. The TCP connection is kept alive for subsequent requests.
2. The HTML references javascript files - some of these are local URLs (locally hosted / bundled JS) and some are from 3rd party domains, like polyfill.
3a. Local JS is requested by having the browser send subsequent HTTP requests over the existing HTTP connection
3b. Content loaded from 3rd party domains (like this polyfill code) needs a new TCP connection handshake, a TLS handshake, and then finally the polyfills can be loaded. This requires several new round-trips to a different IP address.
4. The page is finally interactive - but only after all JS has been downloaded.
Your browser can do steps 3a and 3b in parallel. But I think it'll almost always be faster to just bundle the polyfill code in your existing JS bundle. Internet connections have very high bandwidth these days, but latency hasn't gotten better. The additional time to download (lets say) 10kb of JS is trivial. The extra time to do a DNS lookup, a TCP then TLS handshake and then send an HTTP request and get the response can be significant.
And you won't even notice when developing locally, because so much of this stuff will be cached on your local machine while you're working. You have to look at the performance profile to understand where the page load time is spent. Most web devs seem much more interested in chasing some new, shiny tech than learning how performance profiling works and how to make good websites with "old" (well loved, battle tested) techniques.
