I think it's a misunderstanding that Svelte is fast because it is "compiled" into imperative DOM updates. This is partly because the documentation says so, and the early blog posts emphasized this aspect.

Compilation actually has nothing to do with it. Svelte is fast because it doesn't do all the Virtual DOM stuff, and the DOM diffing. You could very well build Svelte as a library without the compiler, and it would be just as fast.

But the compiler helps a bit with the ergonomics - it's cleaner to do count+=1, rather than setState({count:count+1}).