I did notice this!
I always thought it is because the Platform Behaviour is constantly figuring out angles with SIN and COS, (which makes even old calculators lag badly when using lots of those in one equation), but having it figure out angles, 60 times a second, multiplied by the amount of enemies, it comes to about 600 times a second!
Though that's just an idea, I may be very wrong. Though if right, there should be a way to disable the angle stuff if the angle is 0.