Bazzite uses btrfs and that's prone to fragmentation because any write to a file writes it to a different part of the disk. That additional seeking can make it very slow on old fashioned HDDs. A compromise would be to have your main install on a small SSD and have an extra HDD mounted for game libraries or media (either ext4 or btrfs with a bit of SSD cache in front). The bazzite main system is less than 50GB so any old SSD would fit the purpose.