When I hosted a 10 player game on the Default Escape from stratis mission, my FPS would drop from 60+ to around 20-30 frames. I'm not sure whether its from the Amount of AI or the amount of vehicles, or maybe the mission itself was poorly optimized. But When i make a mission with around 40 AI's in a custom editor, I experience maybe 50+ max. Not as big of a deal.
I have a 670 and my friend has a worse card and is getting a similar drop in frames. It's mostly likely the mission itself and not our graphic settings.