No they aren't slowed down for "human factors" or safety. They are in cages or behind barricades (at least in Europe). If you open them (like a door or cross a laser barrier), the whole line or at least the cell cell just stops and all of the red lights come on. You can believe what you want, but you can't speed anything up significantly without massive investment or work. Or they would be running faster, cause money.

Source: I work in the industry and have been in running car plants.

You can just skip a few welds, don't measure lubricant exactly but do it quickly ±20%, or don't bother smoothly apply sealant. Reduce time to let paint dry. Don't bother with exact alignment. Sure, the car might rattle, rust, break in a small accident and the engine doesn't last long but you're cranking them out fast.