It's debatable.

One of the major analyses on the subject is the poorly named (and in my opinion poorly researched) Time on the Cross by Engerman and Fogel. They make a cliometric argument that slavery was an extremely profitable institution and that treatment of slaves was not as bad as typically assumed. They argue that economic development in the south was greatly bolstered by the practice of slavery due to crops and conditions that would have made it unviable in the North.

It was, to say the least, pretty controversial. Others have made the argument that slavery was not a profitable institution by the 1800s and that it was only held onto out of some sort of cultural hatred.

I don't really buy either view. Engelman and Fogel make a lot of good points, but they get pretty Procrustean with their data at points and try to make much greater statements than is really allowed by the data they have. Also, who names a book that tries to make a historical and economic argument that slave treatment wasn't as bad as we typically assume 'Time on the Cross'? They have some alternate reason for the title, but still... tacky.

Either way, it's an interesting read and worth digging through if you're interested in the economics of slavery.

Economist Scott Sumner argues that Ending slavery made America richer and that segregation laws slowed economic development in the south. This is likely because labor markets were made inefficient, wasting labor on that which was less productive, and making more difficult the transition of labor to accommodate changing economic needs.