Used this command on the terminal: $ sudo sysctl kernel.split_lock_mitigate=0

For more information: https://github.com/ValveSoftware/Proton/issues/6817#issuecomment-1595495268

Hope this helps someone, because I almost refunded the game, lol. It was completely impossible to play online because the game would drop to 30-45 fps on online matches if I didn't use the tinker method described no matter what graphic settings I used (and it was clear from checking nvtop the issue was not the GPU).

Supposedly using gamemode already fixes this by default, but I haven't verified it yet: https://github.com/FeralInteractive/gamemode/issues/425