Its pretty common for multiplayer to be like that for the first several minutes. I'm told it has to do that when one player's connection is slower, rather than lagging them it keeps him at normal speed and makes everyone else's faster. Eventually it tends to fix itself though.

I know, I should have worded that better. It has something to do with correcting the different latency between players. Its not one player's fault per say. Its been widely complained about, there is nothing you can do but prepare for it, it will go away in time.