Don't you think it's the "enabled" enemies that are taking up more cpu than the dormant ones?
Either case you might want to create a destroy and create system where you destroy objects outside the viewport and re-create instances at their respective spots.
Are you using mutiple spawners in your layout? If so you can destroy enemy when exiting layout and save its respective properties (health, magic...) in its spawner and only destroy the spawner as well if the enemy reaches 0 HP. This way, you don't end up with some