It did take awhile for the incompetent union to kill of the republic. It should have been over in a month or two if you compare the two sides militarily/industrially.
I don’t see how you get a month or two before invention of the internal combustion engine and airplane.
Also consider that Buchanan’s sec of War (Floyd) moved large numbers of weapons to southern forts where they could be stolen easily. That act of treason gave the south a running start.
Then figure that southern militias were initally better trained, being assigned routinely to slave catcher patrols, and with a need to be ready to put down a slave insurrection. In the north the militia was starting at a very low level, particularly in the northeast where there was no threat from Indians.
It took a while to train, equip, men, and still longer to train and equip generals and their staffs. Longstreet in particular is known for having the best staff in either army.