I tank DF a lot, its one of my little passtimes. Some end badly others go smoothly. However whenever I enter a dungeon I always do the same thing.

1. I check Peoples gear and quickly estimate their ilevels in relation to the content. I also look at what classes I have because running say brayflox HM speed run with 2 mnks isn't ideal. This gives me a base on what I can push in terms of running an instance. If I see something I don't like I'll slow things down.
2. If I see a new person message flash across my screen I wont speed run, its not fair to them and really I don't expect them to be able to keep up with a hardy group.
3. As a PLD I stuff cover onto my bar. Sometimes I will lose agro just from things like a heal going off to early or a DPS thinking imma stop and begins to go ape shit on the mobs. Cover saves lives XD.

In all honesty if you are tanking an instance you control the pace. If someone doesn't like it they can leave and suffer the penalty or tank themselves.