Gallium HUD
(Source: https://manerosss.wordpress.com/2017/07/13/howto-gallium-hud/)
Gallium exposes a Heads-Up Display that you can use to monitor cpu and gpu load.
For instance launching a game with
GALLIUM_HUD="GPU-load,cpu,fps" PROTON_USE_GALLIUM_NINE=1 %command%
would give the following overlay :
Check out the references for more information.