I have this same issue with especially smoke. When you use for example a smoke grenade, fps drops quite a bit (like 5-15). It doesn't make a difference whether I look at the smoke or not. The fps stays low even after the smoke is gone! So basically when i'm like in the editor and use a smoke grenade, I have to restart the mission for the fps to get normal again. Might just be caused by the bad CPU utilization I don't know.