I had a 12:30 assigned check-in time in Amsterdam last month, with an 11am hotel checkout time. Our hotel was a bit further away, and we arrived at the port around 12. They were taking around 50 people at a time to get into the check-in line, so by the time we got to check-in it was right around 12:30. Check-in took 5 minutes, no line at security, and then we walked right on board.
So keep in mind that you'll want to arrive a bit early anyway, as the bottleneck part was getting to the check-in.