I would suggest, if you haven't tried it yet, that each player loads a single player game before running the multiplayer session. This way the load screen with the art doesn't last long and loading time differences between computers are a bit avoided...
I do that whenever I am going to play multi, and it helps a bit, I still sometimes encounter this problem but less often. However there's no guarantee it'll be working for you... Please try and tell if it works.
Also, Yarlen said :
"This speed issue is by design, as each client catches up to the server and vice versa. It's not something we'll be able to address."
However, Yarlen, if you read this, the concept seems good, but the 'vice versa' part isn't really happening here. On a game with only two players, one server and one client, if one side is speeding up, the other is supposed to be slowing down as I understand. But if both sides are speeding up, isn't there a problem ?
Or maybe I misunderstood you and what happens is the other side instead of slowing down is also speeding up once it realizes it is a bit behind, and so on, resulting in a general acceleration until the game is very fast (maybe when one of the computers has reached his processing limit ? But I really don't know)...