Did you miss this part:

>1) a websocket-based HTML sync layer, basically shipping over the smallest amounts of render state from the back-end. This is as close as you get to having, essentially, a server-side rendered page that is then updated in real time.

Also it's not like you have to load the runtime more than once. So it's not that it's a non-starter for anything performance sensitive--it's a non-starter for anything that can't download a 2MB file up front (I believe their goal is to get it down to half that size) if you want to use the WASM version of Blazor instead of the websocket-based HTML sync layer version.