I have thought about it. So time preference decide the pure rate of interest ( basic rate of return ) and every line of production has the same rate of return otherwise there would be arbitrage.
So if the pure rate of interest is 5% than an investment in a line of production is also 5%, but within a line of production there are different returns on investments. So within a line of production there can be lower, same or higher rate of return.
So slavery is not less or more profitable than any other line of production at that time.
Is this right?