There are already 'express' lanes with maximum item limits, which achieve faster service by making sure the average time to process each customer is reduced. In that case, assigning faster cashiers make sense, but it seems like the 'toll lane' idea would achieve faster service primarily by being much less crowded than other lanes (that is, if the toll lane has a line the same length as other lanes, there would be no point going to it). So having your best cashier there just ensures they spend more time idle, thereby increasing the average time for all lanes.