The company I work for is making a big deal about client-side hydration. We use nextjs for our sites. My director was asking about Svelte.
"Something i wanted to mention is both the React team and nextjs team are aware of this and are working on a solution to address needing to load Javascript on the client. Its called React Server Components
We can try it out today on a platform that supports a node environment. This is from nextjs docs. I have a few thoughts on Svelte, but just wanted to point this out!"
With Server Components, there's zero client-side JavaScript needed, making page rendering faster.
If you're into this sort of thing you should look into Marko. They're kind of obsessed about page load perf with the justification that it's good for e-commerce. They've also been into streaming, partial hydration, out of order rendering, etc for years as part of that general effort.