MAVs are expensive. MAVs on screen are VERY expensive. But worse, they are expensive just existing in the level. How expensive? Well, in my baseline test, 12 MAVs ate up 60 milliseconds of CPU time, not including the actual drawing of them on the screen. Considering I am looking to be at 16 milliseconds for the whole game, this was a problem.
https://bombdogstudios.com/blog/state-of-the-game-225-cutting-a-mav-in-half/

