Looking at the linked paper, it suggests someone was able to make a cash payment in 5 seconds (see table 2). Where the closest of other methods would be a contactless card of about 21 seconds. Just have exact change in hand! Cash also has the highest time of 244 seconds.
Easy enough in places that incorporate tax into the list price, but this is next to impossible in the US. You'd have to know what the city, county, and state sales taxes add up to (accounting for any products that have a different sales tax than others!) and then do the math with the weirdly specific percentage(s) that results (again, taking into account that some items are taxed differently!) to come up with your exact change.
Try doing that with a week's worth of groceries.
Many (all?) European countries include VAT in the price, but prices of 9.99, 0.99 etc. are all too common.
(VAT is a bit different from the way the US-style sales tax for other reasons: https://en.m.wikipedia.org/wiki/Value-added_tax )
It's difficult, but everyone in a local area knows already.