The bottom line is "it depends." First, it depends on how many other ships are in port. Our experience on several cruises is that the
DCL ships get there early. This means multiple tenders--as soon as one fills, the next one ties on and starts loading. We have often had 3 tenders serving the Magic and 4 serving the Fantasy!
OK, concierge guests get priority tender tickets. Guests with DCL excursions meet at their designated spot and are taken to a tender as a group. This does NOT mean that people without excursions wait till all the excursions are gone. With no DCL excursion, you go with your entire party and all your stuff to the designated area (usually one of the theaters). You are each given a tenter ticket as you go in the door. You are escorted to a seat in order of arrival. As space allows, you are escorted to the gangway.
If the tender is there, not full, and there is not a DCL excursions group ready to board, they will take non-excursion guests till the boat fills. If excursion groups are there, they get priority. The longest we have waited was about 10 minutes. The shortest was that I literally sat down and stood right back up again to head to the gangway.
On the way back, it is totally "first come, first served."
When other ships arrive, some of the tenders will break off from the DCL ships and move to the recent arrival. Normally, 2 tenders will continue to serve each ship.
The tendering itself takes 5 minutes. It takes way longer to load passengers onto the boat than to get the boat to the dock.