The founder of Leptos makes a pretty good argument [1] that the bottleneck for WASM isn't really the DOM and that they are already faster than some popular JS frameworks even with the current constraints.
Just tried the site, and already the first problem is isn't as interactive as the Websites for any JavaScript framework, because naturally there is a whole contraption to make the code run into the browser.
That's odd. I have been following along here [1] and it seems just as interactive as Svelte, Angular or any of the others I've tried. There might be a few more tools that have to be installed, but that's a one time step.
Where editing the code, immediately shows the GUI related changes on the neighbouring frame.
While what I see on https://leptos.dev/ is just screenshots, and there is nothing on that documentation you linked that provides the same interactive experience, one has to explicitly start a developer sandbox to host the whole machinery, and even requires a codebox account for editing.
[1] https://youtu.be/4KtotxNAwME?si=IEZ5kRHR_W2o9i_k