The issue of how productive the slave system was in the South is the subject of much controversy among historians. This has been the case ever since the publication of Fogel and Engerman's Time on the Cross in 1974.
The basic argument made against slavery's productivity is that slaves had no incentive to work hard, unlike free workers who would work hard to keep their jobs and to get paid. In addition, the argument goes, slaves would purposely do things like breaking tools as a way to fight back against the system and the people that oppressed them. Because of these things, the argument goes, the slave system was much less productive than the system of free labor.
However, Fogel and Engerman argued that the slave system was run quite rationally by plantation owners. They argue that the system was in fact more economically efficient because slave owners could get more work out of their slaves for every dollar that had to be invested in them. They argue that slavery allowed for the creation of large plantations that could be worked much more efficiently than "free" farms could have been run during that time.
However, this issue still generates a great deal of controversy and is by no means settled.