We've had boarding group 6 for the Wish; boarded around 12:15 pm or so. Boarding the Fantasy we had group 4 and boarded about 11:40 am. I've never boarded later than 12:45; usually, it was open boarding by that time. Even if you are actually boarding at 1 pm, you have an hour to get to the restaurant before it closes (I believe they won't seat after 2 pm, but they'll still serve food), and your room won't be ready until 1:30 at the earliest. We've been seeing 2 pm for room opening on the last 3-4 cruises so as soon as you board, you head down there and don't bother to try to drop off bags at the room.
The last two times, we decided to eat at either the Festival of Foods or Marcelline Market. Lunch at Arendelle had a much heavier menu, and I wanted something lighter. The food was definitely good at Arendelle, but it was just too heavy for lunch for me.