...well, okay, so that depends on your definition of “emulator”, but compared to a more traditional emulator like Dolphin or even Rosetta, there’s very little performance impact from Wine in most cases.
Honestly, it depends on the game. Hence I suggest checking ProtonDB[0] for the games you are planning to play. Most of the reports also have details about CPU, GPU, GPU driver etc.
Anecdotally, I remember playing Team Fortress 2's native port on Intel i7-2630qm CPU on my laptop back in 2012-2013 at 1366*768 resolution with no major performance difference. (setting up bumblebee was not an easy task for me back then.)
Games support Linux either natively or through Steam that leverages Wine (Wine is no emulator), which just supplies the necessary libraries to build the required software environment. There can be a performance impact and some compatibility issues, but you don't have to expect this to be the case. There are even some cases where users report improved performance.
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.