Gamemode
Feral Interactive's Gamemode is one of the best and easiest tools you can use to improve game performance on Linux. It ensures that all of your computer processor's cores are enabled when you launch a game.
Setup
(TO-DO: Gamemode installation)
You often have to manually enable gamemode on a per-game basis, so if you're struggling with performance in a game, make sure to check whether gamemode is enabled.
Configuration
Copy gamemode.ini from here to one of the directories described here. ~/.config/gamemode.ini is a good choice. Change settings as desired, for example to disable composition.