Pages can be server-side rendered and then hydrated after load, in either mode. This makes a dramatic difference in usable latency.
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.