I don't believe, that lesser amount of players play Vanilla Timewalking, than play WotLK Timewalking. Therefore there is only one way to explain ~10-15 minutes difference between WotLK Timewalking queue and Vanilla one - length of dungeons themselves.
There is thread on official forums, where player complains about tanks not completing quests or killing side bosses in Timewalking dungeons. Here is reason why. We have so long queues exactly due to some noobs trying to complete quests in dungeons, while it's pointless waste of time, as dungeon completion grants ~1.5 levels.