Not sure if this is related to the "3FPS" bug or just bad scripting. I created a respawny simple coop mission for my group to play before the "real" missions in our session. When played locally, and (seemingly) alone on a dedicated server, it performs fine. However, when we actually played it with my group, everyone experienced poor FPS, which persisted until their game was restarted.
Mission can be found here - https://github.com/darkChozo/armaMissions/tree/master/Other/fa3_w58_resistance_paros_v1.Altis