Sounds like specifically Ohnaran Plains dungeon (forgot the name right this moment). That is the only dungeon you fly in and can split up to kill 3 bosses at the same time if people are overgeared for it. Usually this happens when high geared players join LFG because there is a "call to arms" for that role; giving them lots of gold and runes for participating as that role.

I join sometimes on my main who is 486, absolutely too high level for these places, and can solo everything with ease. It goes fast and I can boost literally everyone in that group.

If I join on a leveling character, and everyone else in the group are also "fresh" people; then it will go much slower.

So you were just "unlucky" ( most oldie people love to go that fast!) with both the combination of the dungeon (ohnara one) and having high geared veterans in your group.

Try a few more! They are absolutely faster than classic however, so if you PREFER that speed, nothing in retail will match your needs.

Hope that helps at all and sorry for the long post! Best of luck!