Good pc, unplayable FPS (even on potato)

[Leethium]

Hi,
I made myself a new PC last month and the only game i'm not able to run smoothly is Rust.
No matter my settings, I always have about 20 fps

My GPU and CPU usage are very low (30% for CPU, 9% for GPU) , temps are below 50°c
I have no idea where it comes from, every other game runs smoothly

https://files.facepunch.com/forum/upload/316620/1d42a0f2-6c32-463e-88bf-89ede27cea08/RustClient_2018-08-31_13-47-55.png