To do a quick comparison, I pulled up RC's 8 night Norway from Copenhagen. They leave, have a sea day, then they go to Flam (8-6), Geiranger (12:30-9), Alesund (7-9), Olden (9-6), and Bergen (9-5). If you map, the ports on Norway are all really close together, compared to the further distance between most the ports/order DCL is doing (the only two close ports on DCL are Alesund and Geiranger). So going by distance, the travel time between Olden and Bergen (which RC sails) is slightly closer than Geiranger to Bergen (which DCL sails). In Bergen RCI is in port for 7 hours. DCL is in port in Bergen for 8 hours. Both return to Copenhagen the next day, DCLs all aboard time is 4:45 that day, RCs is 5. But, I also don't know if RCs times are the disembark/all aboard times like DCL lists or just the time the ship is in port. If the latter, you have to assume the morning is up to 1 hour later before they can disembark, and the all aboard must be at least a half hour before the scheduled time.
I think we're comparing apples to oranges.
I don't think it's dining or DCL being mean, it's that DCL covers ports spread further apart in Norway, rather than just a small area, in roughly the same amount of time. Now if there were sea days between some of the DCL ports, I would expect them to try to stay later in port.
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ETA, I just checked the Celebrity 7 night. They spend 8 hours in Bergen, 12 hours in Alesund, 8 hours in Geiranger, and 9 hours in Stavanger. In comparison, DCL spends 8 hours 45 minutes in stavanger, 8 hours 15 minutes in Alesund, 8 hours 15 minutes in Geiranger, 8 hours 15 minutes in Bergen. Alesund is the only stop on Celebrety that is significantly longer than the DCL stops. The others are pretty much the same.