PROTON_USE_WINED3D=1 %command%
Fixes slowdowns, especially in Unity games.

PULSE_LATENCY_MSEC=60 %command%
Resolves crackling audio issues. Experiment with different MSEC values (5, 30, 60, 90) to find the optimal setting.

PROTON_LOG=1 %command%
Generates a log file at $home/steam-{app-id}.log

WINEPREFIX=~/.steam/steam/steamapps/compatdata/{steam-id}/pfx wine {any patch or executable}
Use this to install patches, though Protontricks is generally a more efficient tool for this purpose.

GST_PLUGIN_FEATURE_RANK=protonaudioconverterbin:NONE %command%
Corrects missing audio when playing videos.

PROTON_REMOTE_DEBUG_CMD="{executable}" %command%
Launches an additional executable when the game starts. This can be helpful for tools like Cheat Engine and Textractor.