If you're talking about filling up a slow cooker to the brim and letting it roll, sure, you can do that. I personally rather drink Soylent if that's the meal we're shooting for.
For example: If you want to do a stew or chili right, you need to brown the meat. You need to do it in batches to avoid crowding the pan. Batches take time.
And now you're straw-manning. Cooking in bulk generally takes only marginally more time than cooking the same thing for a single meal.