1) Proton labels like platinum, means how much effort you need to put for the game to run on linux. It says nothing about how well it will run. So “work on Linux” means basically that it works on Linux. It says nothing about performance. For instance I had a game labeled platinum run without effort on Arch with 50fps with constant drops to 30 ~ 25fps whereas on windows it runs at a constant 60fps with rare drops to 55/50. (Yeah, I’ve tried all variations of proton, kernel, disabled composition etc)
2) In my experience people that says that they see no difference in performance between linux and windows they usually have a powerful gpu, not a medium or low end one. So if you hear on the internet people claiming that the performance is about the same, it’s highly dependent on the game and/or your system specs.