8-man groups are just 2 4-man groups. 2 tanks, 2 healers, 4 dps.

Up to level 30, healers have pretty quick queue times, but tanks are fastest. After 30, tanks stay very fast, healers drop to the middle, and dps keep the slowest queues. Because dps outnumber healers and tanks by far more than a 2:1 ratio, and healers outnumber tanks.