I had major issues with Doom when It was trying to launch in OpenGL mode. Vulkan mode was perfect.
Quake Live uses OpenGL and it has no problems with rendering and it's performance is on par with Windows.
OpenGL on Proton has worked for me at normal speed with the games I have tried. Using Mesa 18.3dev, LLVM7 and Intel Integrated with i915.
Making Deus Ex (OpenGL) and Prey (2006) to use native desktop resolution with native aspect ratio by editing games' config files fixed issue with those particular games. GLQuake still cannot be launched despite 2 Steam updates and a driver update for Nvidia.
Nothing extracted yet.
As I have just tried with Quake, GLQuakeGL, GLQuakeWorld and Deus Ex; OpenGL renderers crash the games or performs extremely poorly.
System:
Linux Mint 19 (x64) Mate, GeForce 940M (396.51 via Proprietary GPU Drivers PPA), Intel i5-6200U CPU
My Steam - System Information:
https://gist.github.com/rea987/7a5d859effac81a09928b16520f88781