In Sweden people usually start the slow part (entering the pin) while their items are scanned. When all items are scanned you just press ok to authorise the amount and wait for 1-2 seconds for the transaction to go through.
At Aldi specifically it wouldn't be useful as you have to keep up with the cashier's scanning speed while bagging. In other stores you can start bagging at the end. Not sure why they don't use the same idea as Swedish stores. Once I tried inserting my card while scanning in a German store but the machine didn't like it at all.
You only got charged for the amount of fuel taken, so it didn't matter if you said you needed 30 pounds worth and only took 26 pounds worth.
I guess it was similar to pay at pump now, where you enter your card and pin to pre-approve up to 99 pounds, fill up and then only get charged for the amount you took.
Or, come to think of it, it probably lowered the likelihood of people using the supermarket loyalty cards so they nixed it.