Logs would be a good start here.
If this is by design/intentional (since proton/dxvk uses more RAM), then maybe this should be in a FAQ somewhere
my GPU reaches 100% ram usage
So you're talking about VRAM? DXVK is pretty bad at resource management on Nvidia gpus. Performance is expected to be much worse when you're running out of VRAM.
But I wouldn't really expect the games you listed to use more than 8GB of VRAM. That doesn't seem right.
Low(er) 4K performance on a GPU that isn't capable of delivering a playable 4K performance on Windows is not a bug, it's expected.
Intel i7 8700k
16GB
Nvidia GeForce GTX 1070
Ubuntu 19.10 (also happened on 18.04)
Title. Specifically games running @ 4K resolution get around 10 - 20 fps. Every game I have played (non-native) had this issue. GTA V, UNDEFEATED, The Witness, The Forest. Diagnosing with Xserver reveals that during runtime my GPU reaches 100% ram usage. Ram usage remains unchanged regardless of resolution but performance does improve (somehow?). Someone on reddit mentioned it could be this (https://github.com/doitsujin/dxvk/issues/1100) problem but I haven't crashed yet.