That doesn't sound right. I get 80+ FPS on the first mission with the following specs and settings:
1080 TI OC
32 GB RAM
i7-6700k @ 4.5
4K, Highest settings except using SMAA, DoF to High and AO to HDAO.
Do you have the latest Nvidia drivers, 385.41? Those made a huge difference for me.
I don't really believe you on that 80+ fps
From pc gamer with a 7700k

Sub 60 fps is what you'd expect with a 1080ti.
Unless just that HDAO and DoF minor bump down is giving you like 30 extra frames.
My 2x 1070 does a locked 60, and with only one running 30-40, which is right in line with this graph. A single 1080ti doing 80+ frames at near max 4k does not compute. If you have 2 1080tis, sure.
Edit: sorry is SMAA really killing performance that much in this? Eh, either way completely maxed my ocd 1070s hold 60.