ACO is an experimental shader compiler developed by Valve to reduce stuttering in games and improve overall performance in Vulkan applications.
It is included in Mesa by default though not enabled.
You can run any application with the environment variable RADV_PERFTEST set to aco for this to take effect.
In a typical case, this means prefixing it to the command you wish to run. For example, with Games/Supertuxkart, you might run:
RADV_PERFTEST=aco supertuxkart
While for Steam games, including those that run through Steam Play/Proton, you may go to the game's “Properties” screen and set its launch options line to be:
RADV_PERFTEST=aco %command%