Okay, I was able to pinpoint the problem, with the help of my buddies. There is a certain location in Siptah - whenever one of us looks in the direction from max 200m away the framerate drops drastically or the renderer crashes (time out). The Spot is outside the base and floating mid-air. This is the exact location:

No mods installed. Just ordinary Coop-Siptah. Game files verified. There are no player structures very close to the spot but a teleporter - looking straight at the teleporter makes no framerate issues though.

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