Currently using a HD 7770 ( until my "Golden Sample" GTX 660 arrives later this week and pisses on it ) but my performance was better when I upgraded to the latest ATI beta drivers, I would urge anyone to try out them out and if it doesn't work try what Banok suggests and go through some older drivers, going back to older drivers has helped me in the past with some other games.
Anyway, I'm currently playing Pontus campaign and I was taking over "Asia", by attacking one faction called Sardes I had to face the fact he had about 5 allies joining the war with him. However the icing on the cake was that two other local factions then declared war on me , so I'm fighting against enemies from all over right now, it's getting tough but I'm managing to hold out and counter attacking to take regions. Hopefully soon after I conquer a couple of these turds the other Factions will look for peace, if not then I guess I'm going to have a tough time for a long time, considering the Seleucids is one of the enemies and from what I can see on the map they're just as big as I am right now.