150 NPCs on screen kinda just means 150 people that don't know what they're doing walking around the city.
We were having some performance issues with our NPCs in the city level of Vegas Prime Retrograde. We tried a bunch of things, but eventually ended up having to step away from the Unity Navmesh. It's pretty great and full featured, but we needed to really dumb down our NPCs.
We purpose built our own NPC ai in such a way that we can extend their behavior when we need, but until then, they are really cheap on the processor.