In any big group of cyclers (I am using that word from now on - Feel free to join me on my soon-to-be website of
www.CyclersForums.org) there are people who want to go fast and people who want to go slow. The only common thread among them is wanting to not crash into or get knocked over by people who don't know how to ride in a group. The group you found may have wanted to ride at 40km/h, but many others would rather be doing less than half that, and the remainder of people are somewhere in between. Just make sure you are careful and learning group riding etiquette like not using your brakes in a pack, not accelerating off the front, pointing out obstacles on the road, etc.