In reply to Daniel Walmsley.

What is needed on the web is a comprehensive approach to performance, with contributions at the WP Core level, theme level, plugin level, JS framework level, browser level, hosting and more.

Frankly, in the hands of a competent web dev and with a well architected server setup, performance isn't usually a technical issue. It's a business issue. People want excellent design, not just plain text and design imposes some minimum of cost (the use of web fonts, images etc). A site that needs to make money has 2 ways to do so, either the users of the site pay or advertisers pay and if it's the latter, you end up with ad server JS all over (because the ad tech world is insane).

It's nice to talk about a world where this doesn't exist and we should of course strive to minimize overhead, but it's not primarily a dev issue for anyone competent. Some of this are the demands of a real world site that has to satisfy the legit needs of the organization behind that site and some of it is out of control adware that some sites use because they have to make money somehow and ad tech blows at the moment.

PS: then you have bad actors like Youtube who, for a single video embed, shove 1m of JS down the wire and you can't even set it to defer or async.