If you’re experiencing game slowdown or FPS issues the most likely culprit is your installation of Rivals. You can fix this easily through Steam by verifying your game installation like this.

If Steam does need to repair a file, make sure you verify again and receive an “All files successfully validated” message to confirm it’s working.

If you’ve repaired a file with this method and still have the slowdown issue, verify again to see if Steam can successfully validate. If a file is missing again after your initial successful repair from step 1, this means a file is being removed/deleted after you run the game. This can be caused by antivirus software (e.g. Avast or Windows Defender). Temporarily disable your antivirus (instructions here for the default Windows Defender) and repeat step 1.

If you’re able to play with no slowdown now, make a permanent exception for Rivals of Aether in your antivirus to stop it from messing up your installation in the future. Don’t forget to re-enable it once that’s done.