This has been happening to a friend and I in our 2-player games as well. We start in the lobby and for a few minutes everything is fine, but usually by the time we're 5 minutes into the games the clock has slowed so that 1 second of clock time takes 2-3 real seconds. For us it seems to be synced up between us, and we both run slow. We both have slightly older hardware, i7-4790 w/ GTX590 and i5-2600k w/Radeon 6800. We both have fast internet connections (400mbit down, 25mbit up). Neither of us has seen this in single player games, only when we play each other. We started our Grand Campaign not long after the game came out, and I think we were over 1/2 way through before this cropped up, maybe after a patch. We have recently finished that GC game and started a new one, which also has the problem, so it doesn't seem to be related to the fact it was an old save game. We are happy to provide whatever information you'd like to help identify what's going on.