The patch is here

[Wizzie]

That's understandable, the minimum required processer is 1.8, so it'll only really run the game on minimum settings, including army numbers/graphics (night battles are more graphics intensive than daytime battles?).

[Marquis of Roland]

Is there any way to change this aspect of the patch back without having to reinstall? Before the patch, I could play these multi-army battles with only moderate lag so I imagine my computer should be able to handle it, just the game won't let me anymore

[katank]

That has nothing to do with night battles then. I occasionally ran into this myself prepatch.