Unless you are going for screenshots or close up views, unit detail should be kept at low or medium. There isnt much of a difference when you are viewing from the "Eye In The Sky" cam view.
Building detail should also be kept as low as you can go, for city seiges.

Another improvement is to use ">>" fast forward time setting instead of ">>>". Unless you have a super uber computer, large battles are going to be hell to super fast forward. This is because the computer not only has to calculate every variable in the battle, it has to calculate every variable in the battle at x3 speed. x2 speed is much less stressful on your computer.