It's not really emulation. It's running on the same architecture and most of the windows libraries can be used as is with mostly only the win32 library that needs to be wrapped. That already existed for years as wine. It's mostly graphics and peripherals that are broken.

The most important thing proton added to improve gaming was a DirectX translation layer that translates to Vulcan and also loads of fixes and additions to wine.

Not a lot of games run faster but apparently in some situations, the Vulcan precompiled shaders seem to run better than native windows, although that probably means they could make their native version better as well. For older games, the Vulcan translation layer is a lot more efficient and faster than native. Also CPU and IO heavy games might run faster on the Linux kernel.