No, there really is a huge difference. It's more like 1.5 times the y-axis space in your example, they could get 6 self service checkouts in the space of 2 staffed ones:

    >|1|   |2|   <    >|1|      |4|<
     | |   | |         |2|      |5|
     | |   | |         |3|      |6|

Or similar. What does slow things down is elderly or people apprehensive of technology, and tourists or others encountering them for the first time...