Right. But before that mechanisation, slavery still would have been the most efficient means of production.

What makes you believe this? I don't think there's actually much evidence for it in the recent past (previous 300 years, say). Part of the reason that the British Empire was able to get rid of it with minimal fuss in the early 19th century was that it was no longer considered economically viable, and was already in a natural decline in many places.

Most of the resistance to the abolition of slavery in most places was social; it was an upheaval in the way things were done. "GDP will drop" was not, as far as I know, a major concern, nor did it generally happen.

Why do you think this? It adds additional, real costs ranging from the need to finance extensive population controls (with the ensuing social unrest) and an unreliable workforce prone to running away.

Mostly because of the seasonality of work. If I own farmland and want to harvest wheat, I need my workforce only in spring and Autumn. But then I need them 25/8, if you get what I am saying. Buying this kind of labor on a fair market is definitelyire expensive than just "owning" it.