If you memorize the ingredients that the customer requests and make the burger before they come. You can instantly give the burger to them. Be warned as this triggers a bug which gives you a bad rank even if you did an S rank if you deliver the food too quickly.
You must make the burger. Let the customer in and take their order. Go back into the kitchen and return to the customer to give them their burger, that way the bug won't trigger... if it triggers close the game and reopen it.
Btw... this game is peak Chubberdy.