I've tried removing all weapons, stripping all gear, etc thinking it could be something to do with equipment or weapons but no change in FPS. It's also interesting to note that:
_x enablesimulation false;
_x hideobject true;
by itself (without any of the disableAI commands) results in the same or *slightly* lower FPS.
What is still simulating?
One could roll their eyes at such a "performance" issue as most missions don't spawn in so many units (most create/delete during mission runtime) however I argue that createvehicle and deletevehicle sends packets which impacts netcode performance which is multiplied with high playercount.
enablesimulation false objects do NOT send packets and would be much desired IF such a "caching" method had comparable performance to createvehicle/deletevehicle.