Proton isn't an emulator, though shaders do need to be compiled which can be problematic when it's happening constantly. For many games there is next to no overhead -- some games even run better in my experience.
I've had the opposite experience but I'm glad people are having a positive experience in general.
To be clear, I'm really thankful this exists. It's has been super helpful not having to reboot into Windows so often but, in my experience, even with a lot of tweaking things aren't better than on Windows.
