Terrain shadow..bug?

[Hoirtel]

I have been playing in VR with my 1080ti and rift s for a while now. One of the worst performing situations for me is the cold start SC (f18) mission. My usual 40ish fps becomes very unstable and drops out to as low as 23 when the two AI hornets start taking off. Today I got a 3090, testing things out on this mission I got a similar fps as 1080ti. that surprising as I knew that this bit was heavy on cpu. Then I changed my terrain shadows from flat to default (an increase?), reloaded the mission and now have a locked 40fps. Is this a bug? I don't see this performance gain anywhere else but are flat shadows actually more taxing? Maybe flat are cpu heavy and default gpu heavy?