Not necessarily sure about that time of the morning, but the customs facility out at the S gates is rather small, and if there is a flight ahead of you, your wait could be a while. I flew in from AMS the other day in BE and with Global Entry, I went from the a/c to the tram in about 8-10 minutes since I only had a carry on. My slow down happened to be exiting the baggage area, as there were a number of people ahead of me with mountains of bags.