Faster than flat switching, but also a lot louder. The retarders emit a fearsome squeal of metal on metal - especially in cold weather. That's one of the reasons why many of our yards in urban areas have been converted over to flat switching.

Your technology is a bit dated in the explanation above (no more waybills, cars are tracked by RFID tags, switching and humping is done by radio-controlled robots), but the mechanics haven't changed a bit.