The reason why piranhas can strip a large animal like a cow down to a skeleton so quickly is because of a few factors. First, piranhas don't chew. When they bite down, the big chunk of flesh they take out of the cow goes right into their bellies. They just keep snapping their jaws...
“It is a job where your goal is to try disappoint people most slowly.” (source). “So the trick is basically to put them (your direct reports) in charge, not you. You have the supporting role, they can request things from you. But the goal needs to be very clear.” (source) ...