It's obviously dependent on the specific cruise and circumstances, but just a comment that I was assigned for a 10:15 disembark at Barcelona a few days ago, but we asked to get off a little early around 9:45, but our luggage was already ready by the time we got down to the port. Not really sure what their procedures are, but luggage might already be down there for a bit even if you had an assigned time.
Also not sure if OP is trying to get off early or later, but I was able to just get off half an hour before their assigned time and didn't even have to meet in the assigned location - just asked the person at the line to get off the ship and they said we could get off. Might be able to get off as early as the ship allows, if you just ask.