You can preach, but honestly DPS don't have to wait long. I main tank and as someone who does they've dumbed down the role enough to where it's not as stressful as say a tank in WoW. When I don't feel like tanking, DPS ques are RARELY longer than 5 minutes unless you're specific que'ing a dungeon for some reason. Tanks typically don't tend to take long to fill in PF groups for extremes for most fights just because more people play them here.

If it was enough of an issue probably should've just booted the guy if it bother you too much, depending how slow you were going I might have done the same. Had a tank in Dzmael Darkhold single pulling in the first area when they take almost zero damage from the blue zones, I got tired of it and pulled up to boss room myself as healer.

If you're anxious, learning, whatever, just communicate at the start you're new and would like to take it slow, most players have ran these dungeons dozens of times if not more and would rather not take double the time from an overly cautious tank and no given reason