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 alone on a dedicated server, it seems to 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