There seem to be a number of arguments on this according to Capitalism and Slavery in the United States. Slavery as Inefficient was the prevailing argument right up to the 1950s. Beginning in the 1950s the cliometricians argued slavery was productive. There are some who shift focus from agriculture to slave trade. In the 80s and 90s the Marxists argued slavery lacks Wage Labor so can't be capitalism. As one can see it depends on how you define capitalism. Recent trends argue capitalism could not have developed without slavery.
If slavery was productive, then why did the cotton gin become so popular? Automation is always more efficient.