I have the same problem
I THINK it started after I upgraded from 1.113 to 1.125
It would lag at the start of battle, like it was loading something. The game would get very choppy.
After 20 - 30 seconds the choppiness will die down, and everything is smooth again.
This is not lags caused by spawning units. The choppiness usually starts after the troops clash. Before that, everything is dandy. And after some time, it goes away.
Until new enemy / reinforcements arrive, but that is only a one-off thing, not like the choppiness in the beginning of the battle.
But it doesn't happen in every battle, maybe about one in every two or three battles.
So it is not game breaking or anything, but it is annoying.
I tried forcing single threading on/off, load textures on demand on/off, ragdoll on/off, nothing seem to fix it.
System Specs:
CPU: Core 2 Duo 3.0GHz
Memory: 2Gb
Graphics: GTX 260 SP
OS: Windows XP Pro
I would be perfectly happy waiting an extra 5 seconds (loading and preparing stuff) if it means the game would not have to be choppy in the beginning of the battle.