From my limited research and understanding, Nvidia makes Linux drivers, but they’re closed source. These work fine. They open sourced some stuff but not enough to really change much yet.
There are also FOSS drivers, but the performance for those vary.
Is this correct? Should I stick to proprietary drivers if I want consistent performance?
As you remarked, the performance hit you take with the open source driver (Nouveau) is quite varied, but usually significant. The one advantage with the open source driver is that maintenance is generally easier. It can be packaged up and distributed more easily and more quickly updated with the kernel. Depending on your distro, you will have more problems with Nvidia drivers and semi-manually updating with new kernels, though I believe this is not an issue with Ubuntu based distros nowadays.
I don't play any resource heavy games, but I believe that if you are using the proprietary Nvidia driver and WINE/proton or natively supported games, you will not take a huge hit in performance versus Windows.