IMO it's exactly this. Back when I was a teenager I worked at a supermarket and was able to scan a basket of shopping incredibly quickly. But the self checkout machines have so many pauses and errors that it's impossible to do anything at speed.
At Walmart, where I often shop, they have an overhead camera watching to make sure every item gets beeped. Sometimes I get stopped for that. Maybe that system is the bottleneck to faster scanning, or maybe they want to try to avoid double-beeping items and pissing off their customers so they set up a debounce. It would be great if stores were more open about their implementation.
