As another comment pointed out this was the case long before the civil war (which started in 1861 and I was saying you could see this in the GDP difference even in 1800

The South was already set back economically before the civil war. The plantation chattel slavery system hampered industrialization in the Southern US very much in the same way that the serf-owning nobility system hampered industrialization in the Russian Empire. The South, in turn, ultimately lost the American civil war because the northern states were far more industrialized.