That seemed like a good suggestion, so I tried a couple of different battle sizes and unit proportions in both custom battles and the campaign;
- None of the custom battles ever had the stuttering issue, not even with 200 vs 200 cavalry colliding in the mountain pass battle map.
- A ~500 man battle in the campaign didn't stutter, with me having 120 cavalry.
- A ~350 man battle did have the stuttering, with me having about 80 cavalry.
What gets me is that I can't identity the bottleneck here. I mean, it isn't even really lagging---so far it's only ever done that in certain mods (Floris) during huge battles, and then only when my view was focused on the giant mass of soldiers fighting. That's just normal slowdown, and I can understand that; but this stuttering is just kind of vaguely related to large battles.