I have been using a lots more civilians in my NAK Tac missions, because it adds to the realism and difficulty, but having a lot of civilians will add lag to the system and lower FPS.
In a previous post I talked about how to make unit listen to Zeus commands no matter the situation https://www.naksquad.net/forum/viewtopic.php?f=38&t=848, and if you watch my NAK Tac Live stream Mission setup you may notice that I edit the abilitirs of civilian units, so that they do not seek cover, or flee from the area.
Doing some testing today, I found that if you set the civilians skill to 0 it is a quick way to improve the performance, as less CPU cycles are used to compute the civilian actions. I still recommend disabling FSM as well as set the skill to 0.
You can disable FSM using the Change Abilities module of Achilles and you can change the behavior by double clicking on the units group icon. To change the skill double click on the unit and drag the skill slider all the way to the left.
On missions with 20 + civilians in a 500m area changing FMS and skill together had a 15% improvement on FPS. Where changing FMS alone only had a 5% improvement.