I don't know why you would take a chance on the lines when you can get it on line. I agree that "usually" it takes only a few minutes but I have hit it several times in the past (before I knew about buying it on-line) where several flights arrived at the same time and it took 20 minutes just to get the tourist card.
In mid October I arrived at DSQ airport, and I had to buy a tarjeta because my printer at home did not work. There is a machine next to the window where they are selling tarjetas, and a guy next to the machine. The machine only takes $10 US bills, apparently. It will not give change. I asked the guy who was there to make change and he tells me "Se me acab?." Why cannot they get a machine to make change? Why cannot the guy who makes change abandon his useless post and ask the people next to him to give him change? Why make several dozen people wait for no good reason?
It really depends on the flight arrival times. When they are on schedule, the normal pattern mentioned here happens. A bad combination of delays or early arrivals, however, can cause a hellish back up.