While playtesting A17 D3023 x64, I frequently encounter severe lag spikes while entrenched in larger skirmishes. This happens both during the day and at night. I don’t know if the larger total number of combatants is the cause, or what, but I didn’t have this problem when a) my town was smaller (less overall population), b) my military was smaller (fewer Hearthlings fighting) or c) when the quantity of hostiles were smaller.
Feels like an AI performance tuning issue. I’ve included a Perfmon-enabled screenshot while in the midst of lag in case the additional data is helpful: