Slavery definetly is inefficient on the labor level long term. Especially as a generational state that cannot be worked and bought out of. Slaves are effectively encouraged to work only hard enough to avoid punishment and their individual efficency winds up being ignored. There isn't really any reason to try to get better over time except at plotting escape or how to murder every last master around you, even the children.

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.