The performance hit in my testing was fairly signficant. I saw improvements with different versions of Proton, but ultimately I was looking at a 5-20% reduction in performance vs my testing in Windows 11 when playing more demanding titles. There were also frustrating anomalies where games would randomly refuse to start or would crash. Yes, you can certainly use an Nvidia GPU and get decent performance out of older titles, but there are still some fairly significant concessions that need to be addressed. This is well documented among the community.