At peak hours it can definitely be a problem, you really need the entire pipeline to work well. There's going to be somebody behind you most of the time, and you really don't want people stuck at the gates.

In Japan credit card transactions routinely take a couple seconds. Imagine each person taking 5 seconds to go through the gate! I think what trials for credit card payments in transportation services there are doing is simply not processing the transaction inline, and just doing it after the fact (assuming it will go through).