Kalarhan gives great advice as always.
Just to add to the configuration info at the post he linked, there are some VC game options (blood loss and fatigue) that I think slow things down on big battles as well.
FPS is also hugely affected by the direction you happen to be looking in. If you face the enemy shieldwall and can see all the units, it goes way down compared to skirting the edges. Not always practical, and big battles are fun to watch, but sometimes facing so you only see a part of the armies helps a lot.