IME it's indistinguishable. Often the performance is slightly better, which is shocking considering it's a translation layer.

Especially in GPU-bound titles, there are endless examples of Proton running better than even native Linux versions. Here is the venerable DF Direct crew comparing Cronos' recent Linux-native build versus Proton: https://youtu.be/Sj5TyrHDspU?t=2951

DXVK is remarkably optimized and I think many people overlook that.