I have performance issues too but I think I found the cause.
I created a little script to monitor processes running on my CPU and when I start the game "Oregon.exe" shows up using 200+-%. However the "Oregon-Win64-Shipping.exe" is run by the GPU (using 1783MiB in main menu 4000+-MiB in-game).
I tried to force the game to use my dGPU, using launch options, modifying .ini files, enabling performance mode and even to disable iGPU(resulted in black screen), but nothing worked. Unfortunately Nvidia X Server Settings won`t let me choose which GPU to use like Nvidia Control Panel in Windows which is how I fixed that problem on my former Windows PC.
proton 8.0-3cx1 2023-08PROTON_ENABLE_NGX_UPDATER=1x1 2023-08PROTON_ENABLE_NGX_UPDATER=1`x1 2023-08PROTON_ENABLE_NVAPI=1x1 2023-08PROTON_HIDE_NVIDIA_GPU=0x1 2023-08
Compatibility Report
System Information
I confirm:
steam-1583230.log
Symptoms
Reproduction
PROTON_HIDE_NVIDIA_GPU=0 PROTON_ENABLE_NVAPI=1 PROTON_ENABLE_NGX_UPDATER=1 gamemoderun %command%(assuming your system already has FSYNC configured properly) - (I personally havePROTON_ENABLE_NGX_UPDATER=1set in/etc/environment)symptomsabove, and set the game resolution to run at 3840x2160)