I'm not entirely sure what you mean, but I've had (have...) similar problems. Sometimes, if the first battle I fight is an attacking battle, the game just won't cooperate and the game is so slow that I easily have time to make myself coffee while the bar between the campaign map and the battle map is loading. Once the battle begins, I have to exit immediately (earning my general a Weak Attacker vice or similar) since the battle is just too slow to be playable.
I once tried to fight a battle despite the "lag". It was a very easy battle with my 40 Chiv knights against some 60 UMs (newly conquered province that rebelled), so I just ordered my knights to charge. The whole thing took about 30 minutes...
The second, third etc. battle always work fine, it's only the first one that screws up. I have no idea why really.