1: Actually, that's not true! I use Leptos in their "islands" mode for server side rendering. The entire page is sent as an HTML response, and there is little/no "hydration". The WASM file ONLY includes interactive "islands" that are explicitly marked with the #[island] attribute [0]. In other words, the server binary is handling most of the rendering, similar to if I used a templating tool like Askama or Tera.
2: Leptos is generally slower than vanilla JS, I believe for that reason, but comparable to major JS frameworks [1, 2].
1. The browser needs to load the whole app before anything else could be done resulting in a slow first load.
2. WASM -> DOM manipulation is slow.