What makes someone fast?

What do you think would be harder to accomplish for your average joe, fattest or fastest human. I’m going with fattest because I think almost anyone could get there with years of stuffing themselves with black and white cookies and such, but very few people have the genetic talent to be the fastest ever. Then again, I don’t know how many people could live through going from normal to the fattest ever. What do you guys think? Are both near impossible for the average person?

How athletic were Ben’s parents does anyone know?