> then arguably some form of webcomponents might be theoretically better optimizable by browsers than DOM-element soups.

Arguably they might be less optimisable than regular web frameworks, as they make it difficult to coordinate between different components on the page.

This is a good point, but I would counter, perceived speed of rendering from a user perspective is more important than time to interact with the app/site (a reasonable time to interact is however very important.

Good load order and only loading in the content and JS above the fold initially helps a ton.

JS frameworks are huge and I am still saddened that proper tree shaking and optimization got left by the way side in how the modern web has evolved. I worked with google closure in advanced modes and the closure components for a few years and the compiler was absolutely astounding (and the components designed with the compiler in mind) in code splitting, tree shaking and minimization.

Oh, google closure was absolutely ahead of its time!