The poor design of the tray system is definitely a contributory factor. There simply isn't enough space on the conveyors before or after the scanner for the average (non-FF) to sort out his/her things without holding up the line.
Secondly, it isn't obvious what to do with the trays. Ok, there are signs, but most people are focussed on grabbing their stuff and running. Given that the staff don't pile up the trays, this creates a problem. If you had a reverse conveyor, like at EDI, it would be obvious that you put the tray on its side, for it to roll back to the start.
There are so many design flaws at T5, I don't know where to begin.