Yup. It's the exact same here. I thought it was just my system at first but I don't believe it is now. I think there is some issues with these battles causing the game to slowdown.

My specs:

3.6 Ghz Intel Dual Core
1GB of RAM
Geforce 7600 GT

I just don't think this game is as well optimised as Rome was (at least in the latter days). Let us hope this is resolved.