Small update - the game does not suffer this issue in the OpenGL renderer.
Hello @SteveHeist, which version of Proton were you seeing this issue with?
Proton 3.16-4 Beta @kisak-valve
Doesn't seem to happen on Proton 3.7-8 Stable
I have this issue with proton 4.11.x with nvidia.
proton 4.11x1 2019-12proton 3.7-8x1 2018-11proton 3.16-4x1 2018-11
System Info: https://pastebin.com/t6CreLgS
System Info Brief:
Ubuntu 18.04.1 LTS
Ryzen 5 1400
RX 580
Mesa 19.0.0 drivers (Padoka PPA)
16 GB DDR4-2660 Dual Channel
As stated in the title, looking around with the mouse causes the framerate to drop to refresh rate (75 FPS). Stopping mouse movement (ie, not looking around anymore) allows the game to reach 150+ FPS in the Vulkan API.