The percentage on the top of the screen can start decreasing slower on the earlier shifts and ramp up as you progress (much like the Warioware games do).
Also, I could barely tell when a "perfect" or a "so close" made any benefit to it (is it supposed to?), maybe the bonuses/deficits for serving the customers well/badly should be clearer.
Alternately, it would take more reworking but each customer could have their own timer bar and either a clock indicating the end of the shift or a number of chances that will decrease if you mess up an order
The cheese refill mini-game seems to be useless for me because the appropriate window happens too quickly for me to react, so it could be another thing that ramps up over the number of shifts progress.
Overall I like the idea of it so far, but right now it feels like a git gud score chaser which is way less amusing than it seems like this game is trying to be