for someone like me 2.5-3 hours the computer is a little too.
It is a problem, the time needed for MP games, but over at NBC we are playing 8 hour battles like this :
The early phase at 10/60 speed.
The main, longest section, at either 15/60 or 20/60 depending on how much time different people have to spare.
The end phase (ONLY if it's possible, and sometimes it's not) back to the 10/60 speed.
Using these time speeds we get most 8 hour battles done well within the 3 hours, no problem at all.
It helps a lot if the host is ready with the game set up, map, OOB and doctrine. This avoids too much "wasted" time at the start.