Foreman was actually a fast puncher in his very early youth, compared to as a WC, what happened?

[Pugilist89]

Fast forward to his bag work. His speed is not bad. Why did he get slower when he became a seasoned pro? 2:40 it starts when he's very young. Watch the full. https://m.youtube.com/watch?v=sLEg3gQBBRE