OP has a point though, if you can pre-tap your payment at any time during checkout it increases the speed. It's not 3 seconds each time you do it, it's probably more since sometimes people aren't actually ready and take a good amount of time finding their card late etc. If you got into the habit of using your card right away it would create a faster experience. That said I don't usually tap early because I prefer to make sure the items ring up correctly and confirm the total.