Build number: 8774.1520175
I randomly get some pretty bad lag during battles and I don't know what causes it exactly. I have seen it happen when my archers loose a volley of arrows, but not every time. Never seen it happen when artillery fires either. Panning the camera away from the battle sometimes helps, maybe it is large unit size?
I usually play with no browsers or other programs running in the background.
edit: forgot to say that it occurs regardless if I am using mods or not