(or so he was told. Often. By every Hollywood director ever.)
http://www.factfiend.com/bruce-lee-literally-punched-fast-people-react/
He had to be slowed down TWICE.
Firstly he himself had to reduce the speed of his motions.
Secondly even after the above the film still had to be slowed down.
Otherwise you could not see him move in the final film. And he looked like he had superpowers or something (https://www.youtube.com/watch?v=k3jt5ibfRzw#t=41s)!
Myth NOT busted. Bruce Lee really was too fast to be human.