A couple of things might be in conflict. First is efficiency before fatigue sets in. It's possible that there is one position for my feet on the pedals which gives me the most efficiency. But holding my feet in that one position can bring on fatigue. Fatigue reduces efficiency. At that point, I'm better off moving my feet than keeping them in one position.

I still use clipless, but sometimes, I unclip just to have a different position.