SPA = single page application. You can make one entirely in Blazor and can host it on any static file host. Are you talking about Javascript specifically?
Blazor Server: On a broken/poor connection there is no interactivity anymore
Blazor WASM (=SPA): Keeps working when connection is lost, but everything is sooo slow
classic SPA: Keeps working with lost connection and is fast.
Why should it be slow? Once Blazor WASM is loaded, it doesn't need a connection to the server anymore.