I believe the reason for the tiny shelf you have to quickly empty is twofold, it means the tills take up less space, which means more shop space for stuff they can sell, and it speeds up the flow through the tills because peer pressure makes most people just pile it straight back into their empty trolley and sort it out after they've paid.

My strategy is to load everything onto the belt (heavy stuff at the front, fragile at the back), and line the now empty trolly with two or three of those strong reusable bags they sell, I can keep up with the cashier and end up with well packed bags that are easy^h^h^h^h possible to lift straight into the car, but then I'm a bit nutty when it comes to packing things, bags, cars, dishwashers.