Maybe Using pre-rendered 2D Sprites for enemies and calculating "AI" for 5 enemies together making them 1 enemy (just for the movement calculation) and then offsetting the values. Also: Maybe your Engine has some kind of "LargeGroup"-Optimizations for Rendering. But I'm not I pro at things like this. These are just some guesses. Maybe make a backup and try different methods and look which enhance performance the most.