The article or the ProtonDB website says that 80% of the top 100 games from Steam are at least Gold++. For his group Gold++ consists of 51% Games with Gold+, meaning that the run as good as on windows and additional 29% that run native on the platform. So your performance should be as good as on Windows.

IIRC quite some games did run faster on WINE (if running at all) than an Windows. I would be interesting to know if the same is true for Proton.