Ignoring the obvious humour... although most of the vitriolic chest-beating in this thread is born of ignorance and arrogance, calling and indeed thinking of the individuals on the checkouts as 'monkeys' is actually very offensive.
To be fair, I've referred to myself as a till monkey a few times. Not entirelly sure where I picked it up from, but I just see it as a bit of a laugh, its a fairly menial job afterall. Though as you say, some people could be offended by it.
Whichever jobsworth devised such a system deserves to be punched in the face, repeatedly until but a bloody gelatinous mess sprinkled with the odd fragment of bone is all that remained.
For some reason, 1600 an hour stands out to me, but doing the numbers makes that ~27 items a minute, which seems pretty high. Its not supposed to be a super fast target which requires you to just fling items down the belt at people as they're packing, but it also encourages you to scan and pack if you see the customer is struggling to keep up. And gives them an obvious reason to get rid of people who're way off the target.