Comment by lopmotr

Although reducing speed and fuel usage reduces fuel costs, it increases fixed costs (you need more ships for the same throughput) as well as operating costs like crew. It might turn out to still be more expensive in the end. Then there's the problem of less certainty about price changes if you have to wait longer for the product to be delivered which leads to inefficiency (over/under-supply if mispredicted). There's also the opportunity cost of money being tied up for longer in the delay between production and consumption.