It's still easier and faster with a group unless you are just taking a bunch of dead weight players with you. Having 3 people in a quest doesn't give the mobs triple hit points and triple damage compared to soloing. Look at some of the insanely fast completion times people can get with a group of 3 or 4 people that know what they are doing...it isn't even physically possible for 1 person to do the quests that fast.
If you are talking about pugging quests, well, pugging has sucked in every MMO ever made and probably will take longer than just soloing which is why a lot of people solo if none of their friends are online.