Last night a group of us were up in Svetlojarsk (top right of map). We had two styles of weapons. AKM's with the PSO-1 scope, and M4s with ACOGs. When we would look down the scope in the city (we were near the police station looking around) it would drop our frame rate to the point of almost 1-2 frames and cause us to lock up for a bit.

It happened in this city almost consistently (about 85% of the time). Once we left the city, it was fine.