Why do my legs burnout at lower speeds?

When I run on a treadmill I've noticed that I will get tired and my legs will start burning more if I am at a slower speed like 6.5 ish than around 8 or 9. If I am running at 9mph I get can do a 6-8 minute mile without too much discomfort. With 6.5 its more of a 10 min mile with my calves screaming at me to stop at the end. Am I alone on this? I'm 6'1 185lbs if that helps any.