1) According to the economic theory, forced labor is never efficient. How was slavery beneficial economically during the transatlantic slave trade?

I added during the transatlantic slave trade to make the question more specific, rather than it just meaning slavery in general. I plan to talk about multiple centuries throughout the paper, so I didn’t include a certain chronology time in the question. I plan to use databases provided by the school, and I also will use google books and other websites for research. Hopefully I will also be able to find some primary sources too.