I meant in general: I'd expect moderately happy slaves to perform better, and the cost of keeping them moderately happy lower than acquiring new slaves over and over; creating slaves has a cost.
Accurately answering your question requires writing a thesis: one needs extensive access to accurate data spanning thousands of years, a solid grasp of history, psychology, ancient customs, etc. Those situations are full of subtle nuances; what historians currently understand might not even be that accurate.
OTOH, casting reasonable doubts by assuming a fair amount of people weren't too stupid is less bold of a position than "slave owners were living devil", but at least it's honest.
(Which doesn't imply that "slave owners were living devil" isn't true, merely that it's dishonest to say that it's true, because it's too difficult to know for sure).
Slavery is an old thing[0]. Even assuming the death rates are correct, one can't honestly conclude that what happened in the West in the past 500 years is similar to what happened, say, in Antiquity in the West[1] - and that's what most relevant to Aristotle - or amongst ancient Jews[2].
I know that there are too many unknowns for me to even have a clear thesis to begin with.
Are you kidding me? You think the slaves being worked to death in the Roman silver mines or galley ships had it any better than the ones in the New World? Pre-modern slavery was just as brutal.
You seem curiously attached to the happiness in slavery fantasy.
Let me repeat myself a third and last time: I don't have enough data points to conclude; acquisition of enough data points to reach a honest opinion is to daunting of a task.
It's not because I don't share your viewpoint that I share its exact opposite either.
What I think is happening here is that your image of a typical historical slave is a mildly treated house servant. >99.9% of slaves have either been toiling in brutal conditions involving large agricultural enterprises, mining, or construction projects (men) or forced into sex work (women). The latter is actually more evil.
Even in the case of the house servant you imagine the typical slave to be, you have not considered factors like having their children sold away from them, or the death of a kind master leading to being sold or inherited by a wicked one.
What's the cost-effectiveness of a master selling one of his female slave's children away from her, which was a regular occurrence?