On-line check-in and "fast" bag drop are concepts that were designed before the "one carry-on per passenger" rule. I believe one of the contributing factors too the seemingly lengthier queues at bag drop is because more people are having to check in bags - people who would ordinarily just check in at home and go straight to security.
Another factor is the amount of passengers who simply will not conform to the luggage allowances specified in their booking and will insist on arguing their case at the bag drop desk. This ties up check-in desks and is frustrating for the agent and the passengers waiting behind in the queue.
Management of these two factors would certainly make a big difference in waiting times.