The xmid definitely has a learning curve. It might take 5-10 minutes the first few times you set it up, but once you get familiar with it, I'd say it can be set up in 1-2 minutes and could probably be done in less than a minute if everything goes flawlessly.
I've also learned more recently that you don't need a flat spot for the entire footprint of the fly. It only needs to be the size of the inner, which is only a couple of inches longer and one inch wider than the double rainbow. However, you won't get a tight, pretty pitch on sloped terrain.
Looking at the specs, the double rainbow has a 98" x100" footprint and the xmid 2 - 85"x 100", but that's measured at the widest point and the rainbow is a hexagon, while the xmid is square.