(as far as I can tell, I don't really understand what Prodosy is)

Prosody is always running, whether or not you have it configured with a password. So I'd guess zero extra load.

Additionally, without a password your server is wide open, letting anyone start their own conference calls. I would guess that load could be significantly reduced by stopping unknown people from connecting.

(I still don't understand jitsi meet's architecture well though, so there could be ways around the password)