Dude if it takes 30 seconds to make change that cashier has some serious mental handicap going. Easily 2, sometimes 3 cash customers turned around during your 30 second transaction. Assuming the cashier isn't a complete idiot or some customer isn't mining the bottom of a giant purse for loose change.
So, I see you pay as much attention to how your typical cashier is lacking some basic skills as you do to how tipping works. This ain't like the world pre-90s when figuring out change was part of the education doctrine of the time. Current cashiers are better off pressing up the sale and waiting to hit a confirm button on a charge.