How fast or slow you go isn't determined by flow. That's like saying unloading fast or slow is determined by how many packages are in the truck you're unloading. It is entirely relative and up to the worker to do "their best" while maintaining some semblance of safety. If you are loading @ 250 in a truck that "requires" 450, you either fall for the manipulation and work faster, get help, or are moved to an easier area (despite what they would want you to think).