Indeed this is something that even George Washington famously wrote about. His conclusion was that slave labor is an economic disaster and that he would be far better off financially not being responsible for carrying all the costs of the labor. And that's before the US developed a far superior economic model to what existed in Washington's time, in regards to productivity (ie even in a backward, low productivity agrarian context, it was obvious slave labor was extremely inefficient, it did not work well at all).
This is something that has been proven repeatedly by studies that have looked into slave labor in the colonies and elsewhere. When people on the Internet say otherwise, my experience has been that they never support their claims, they're always empty statements held up as fact.
The US would have developed faster if the slaves had been free. Along with being obviously morally evil, slavery inherently must involve a great misallocation of human resources. It's an extreme example of command economics. The Soviets, Chinese and others more recently have demonstrated how poorly slave-based economics works in practice. We're not lacking for proof; there isn't a single example from modern history of it working well versus free labor.
The greatest example of this in action in recorded history, is modern China. They only developed at all after they began to shift away from a de facto slave-based labor system, to something closer to free labor. Simply put, they unleashed their human capital and it has done the rest, going to work building out modern China (in spite of the restraining, backwards, command economics system that remains, rather than because of it; something Internet pundits frequently get wrong about China).