Sorry, what? Slavery was never "inefficient." The problem is that humans have rights and holding slaves deprives them of those rights. To the contrary, slavery was, is, terribly efficient for the slaveholder.
The really fucked up part is that the advent of slavery was an improvement over stone aged bands warfare of genocide where at best some of the women and children might be forced into concubinge and human sacrifices to dispose of unsustainable mouths to feed.
Wonder what he would say to the mechanization of labor through technology and its benefits of ending the institution of slavery and providing owners cheaper or eliminating labor expenditure?
So just another view point on the issue of "inefficiency of slavery" argument.