Have you tried the performance profiles?
And in terms of the map size, that's not really the factor if you're having slowdown during battles -- it's the CPU. Or, somebody's CPU. Check out the thread I linked to above, as it has methods for determining if you have a graphics, network, or CPU bottleneck, and who it is that is having the problem if there is a problem. The game only runs as fast as the weakest computer can run it, since they all have to run it simultaneously, so it's entirely possible that it's not you with the problem.