Hello @4lphac, we're using one issue report per unofficially supported game title, and issues with native linux games are outside the scope of this issue tracker, so I've gone ahead and transferred this issue report to https://github.com/ValveSoftware/Proton/issues/135#issuecomment-629806754.
That said, if native linux games are equally affected, then that hints the issue you've encountered is outside of Proton.
Compatibility Report
System Information
steam-356190.log is 156MB!!! wtf?? Cannot attach it, there's for sure something goin on..
I confirm:
Symptoms
It happens both games (first with steam linux version, second with proton) shadow of mordor and shadow of war, the game speed turns "uneven" after a few seconds of playing (1920x1080 details to lowest) , it begin going from very slow to very fast (like someone turining from slowspeed to fastspeed a fictional knob), and back again, endlessly, in the second title it is so persistent that the game is unplayable. Note that there is no stuttering, internal game benchmark gives 82 fps (without RADV_PERFTEST=aco) and 98 fps (with RADV_PERFTEST=aco) and has no trace of that cyclic speeding up and down. It's the first time I see something like this, very strange.
Reproduction
I think it's very much hadware/driver related