Thats a very good point and honestly dont know, lol
However in my experience my two definately get more tired when we have been off lead. 15 minutes on a lead doesnt make a difference, however 15 minutes running around a field/garden constantly will. A 15 minute walk for me might be a mile on a lead, off a lead i will still walk a mile but they will run more; back and two.....
Again, this is in my opinion