As a concierge level guest, you are #1 boarding no matter when you arrive. This means that you will check in thru the concierge line (shorter than the normal lines), have a special waiting area with seats, and will be in the first boarding group. There is no "order" within the groups....it's just who gets to the boarding line first, but you are talking about maybe 2 minutes between the first person in group one and the last person.
SO...boarding usually starts about 11:45 although the written literature says later. If you arrive at 10, you will get to sit for 90 minutes. Yes there will be brief character appearances, etc. but reality is that you will sit and wait. My suggestion would be to arrive in the 11:15-11:30 window. That gives you time to check in, and regardless of arrival time you will be one of the first on the ship. My experience with returning a rental car has been VERY positive, with little or no waiting for a shuttle to the port. In fact, on 2 occasions my "gang" already was on the shuttle ready to go when I came out of the building from the car return. In both cases, the shuttle was there, and the driver suggested that they not even go in the building with me but just get directly on the shuttle. In one case, I had 5 teens with me (and the luggage). We had planned to drop luggage at the terminal, but when I found out that Thrifty was literally across the street from our hotel, I decided against the "extra" trip down the road to the terminal.
If you were not concierge, there are 2 good arrival times. You'd want to arrive either early (10-10:30) to get a low boarding number, or arrive about 12:30. By this time, the people who arrived early have boarded and the bus traffic from WDW has not arrived yet....so you would check in and walk right on. The WDW bus traffic arrives about 1:30-2, again making this a crowded time.
All that said, the check in lines are much quicker and shorter than in past years as
DCL is working to develop more "off site" check in to prepare for the Dream. Guests are being allowed to check in at the Welcome Center at MCO and at some of the resorts (I can't say whether this is happening on all cruises, but it has happened to us at both places). You do get a boarding card at those check ins, and security photos were taken at MCO. Thus, there is nothing for those guests to do at the terminal except board the ship! NICE!