What? No! The game does not slow down. Decisions just will have to be made on the spot, locally, by commanders marching with the army. And no one would be getting fewer messages. Everyone would get the exactly same amount of messages, just with a delay.
You are assuming that a) people on the spot are both willing
and able to make those decisions, and b) that people will keep sending as many messages, when they know they won't hear back for days.
Neither of those are safe assumptions. Even in BattleMaster's heyday, with over 2000 active players, they would not have been safe assumptions.
