Also historical records show many slaves were treated horrifically. Other than seeing them as subhuman and treating them as such to reinforce their own ideology, slaves could be killed to set an example for other slaves (or tortured), such as if they escaped, refused orders or something like that. The reason wasn't to get more work out of that particular individual slave, but out of all slaves, be it in society (such as if they tried to escape), or if there were in a big group. And yes slavery inherently produces less value than wage-work, by a pretty wide margin, so by it's very nature it produces less. To argue that, they wouldn't do something that would result in less production of goods is frankly, pretty silly, since slavery itself does this. On top of this, they deliberately would shackle them, prevent them from learning how to read or generally be educated, and whip them which lead to infections and blood loss, just as a matter of course, which in no way long term is a good thing. There is such a thing as just being stupid and inefficient, and the nature of slavery leads to less productive labor in the first place, hence why they continued the practices.
