Why did George Foreman punch so hard despite having lack of speed ?

He had big bones and big tendons...if you add all that weight (mass and fat) + height + rage + resistance to pain...you get the puncher he was.

 

"It doesn't matter how the fast the train is going when it hits you." Sonny Liston

 

Foreman had speed, it's just that he telegraphed many of his punches, and when he was exhausted he was slower and ineffective, he was often swinging wildly. But the velocity of his KO punches was legit.

 

This is also the strength component, helped by his bone and joint structure. Foreman was extremely strong, and one of his often used tactics was just shoving the opponent across the ring using a single arm. When you are strong enough to push through the opponent with your punches with little effort, you are transferring all the force you generate into the opponent. When you have this strength, punches that land from weaker angles and indirect blows can still cause damage.

For example, a strong fighter like foreman can punch at your guard and smash through, while throwing your body all over the place. Beterbiev and Wilder can also do this.

A physically weaker fighter like Pacquiao had to land perfect punches and pierce through the guard to do damage. When he was wailing on Broner and Floyd, you will notice that their arms and bodies didn't move at all when Pacquiaos punches landed.