I was under the impression RTS games were synchronous because doing client-server with multiple hundreds of discrete units in existence would require a lot of network bandwidth. The game is "held back" by the slowest machine in synchronous models because the game state would de-sync otherwise, as all that is being sent over the network is player input, rather than every unit's state. Maybe my understanding is wrong, but the "heavy lifting" being done on a central server is not very important in consideration of the required up/download to and from every client.