Many of the contracts relating to bag delivery are about the first bag rather than the last one so first bag out after fifteen minutes is normal and if they get one on the belt then they've complied. That you are still waiting for a priority tagged bag after an hour is irrelevant to the SLA. It would be much better to see a SLA that had priority tagged bags out in fifteen minutes and the rest in thirty but I doubt it will happen because costs would go up.