This whole circus really is a culture clash. A bunch of spoiled Westerners telling us central Europeans that games we make about our lands is as baffling as if someone was screeching at people that the sky isn't blue. It's a good thing Vavra is well-versed about the world that he can defend himself properly, most people would just be dumbstruck.
It was efficient in its day but wherever it was abolished those places would actually get a boost in economic productivity. The industrial revolution started on a small island with no slavery after all.fftopic: scuse you, wot
Slavery is an economic powerhouse of a system, more efficient than indentured workers or regular farmers
Of course the higher up in details you go with it, the less viable it becomes because you'll have to teach the intricacies of the job to each slave, but for menial labor and general small - medium tasks it's good (barring morally)
Physical coercion is one hell of a way to make sure your workers don't strike on you, you can make them work longer hours for less pay, and they start working at a young age. I'd say that'd be pretty efficient for most.