Interesting responses. I wonder if the Kenyans were able to run more comfortably at a slow pace because they did it more regularly, and their muscles were better conditioned for slow or fast paces.
In my youth, I was occasionally accompanied on easy runs by a variety of Sub-4 guys (3 as it turns out), and not one ever mentioned discomfort, but after running just a couple of miles with a much slower female friend recently, I began to wonder how bad it must have been for them. At about 2 minutes off my typical easy run pace, my quads and ITB were sore and uncomfortable.
The comfortable pace difference must have been at least 3 minutes between me and those Sub-4 guys. How sweet that they never complained.