From a factual perspective, the parts of America that were free states did much better in terms of creating wealth than the parts that were slave states. The North became industrialized and generated much more wealth. One reason the South lost the Civil war was that they could not compete with the armaments, etc., that the northern states were able to bring to bear due to their greater wealth.

Slavery is simply a p*ss poor economic system because, like communism, it ignores the profit motive.

Compare East and West Germany prior to the fall of the Berlin wall, or North and South Korea... under communism, people know that if they work harder they will face resentment from their colleagues and get no material reward. So they do the bare minimum to get by.

Similarly, in slave economies, human potential is wasted. Why should a slave work harder than absolutely necessary or share a creative idea that he or she came up with?

Southern masters often complained that slaves were naturally lazy. The reality is that the slaves were not lazy, they were exercising the only rebellion possible in their powerless state. If they actively disobeyed, they would be punished. So they passively disobeyed, by working hard enough not to be punished, but no harder.

When slaves were set free and allowed ownership of the fruit of their labor - then they were more productive.