Cheers! The time it takes for a new customer to spawn is actually decreasing as you play, in order to make the game harder and harder, until impossible at one point, like those old arcade games that inevitably end in failure, though it's a bit less punishing in this game, because you still have a chance of getting 5 customers wanting drinks in a row.
But I might see if it needs any change, the whole feature was definitely a bit rushed.