One thing being a boss of hourly workers for 15 plus years has taught me is that work, much like a gas, increases in size to take up the space of its container. Translation: If I have 4 workers and 1000 orders to ship today they will ship 250 orders per person on average. If I have 4 workers and 800 orders to ship today will ship today they will ship 200 orders per person on average, with no guarantee that they will have extra time to do anything else.
How is this possible? I think it's just human nature not to push ourselves. Why is your last day before a 2 week vacation so efficient? Because it HAS to be. You have things to wrap up before you leave.
Recently we've had a number of our fulfillment workers leave for various reasons, and we haven't replaced many of them. I don't think I'm going to replace most of them, either. I've seen what they can do when they have to, and quite frankly I have not figured out a way to get the same level of productivity on a day in and day out basis as we have on a busy day. I have some ideas how it could be done with software, but that would require a "one system to rule them all" that would encompass a time clock, our picking software, our shipping software, etc.
Note: This approach only works when there are clear goals. If there are no clear goals for every single day chances are you won't achieve as much as possible. It's like the old Yogi Berra quote "You’ve got to be very careful if you don’t know where you are going, because you might not get there.". This is an area I need to work on with our marketing department.