This is why I liked the cashier speed. Some cashiers are consistently below 80%. That's not because they can't game the system. That's not because they got unlucky and had a few slow guests. That's because they put no effort into moving quickly (or don't have the physical ability to do it). These are the ones that cause more lines (making guests wait more) and pulling extra people for backup (making people do extra work because you aren't fast enough to do it yourself).
I will say that my store approaches cashier speed in the worst way, mostly thanks to my etl-ge. We coach the people who have red scores and that's pretty much it. If someone has 10 transactions and 2 are red, that score is 80% and we think they suck. Then we have cashiers who have 500 transactions and a score of 85%. That looks better, but it has an enormous affect on the overall store score.