I have yet to see a convincing argument as to why I should enable a tank to go slow or match their pace if it's going to drag the instance to 30 minutes. If someone is new then a minimum of 2 packs is safe enough to be comfortable and for me to not be bored out of my mind, and we'll still clear in under 15 usually. I will not enable new players to develop bad tanking habits if I encounter one in a party because that is the equivalent of kicking a can down the road for someone else to deal with.

If they cannot adapt, then I leave because I am under no obligation to carry someone who is either unwilling to improve their gameplay or too clueless to do so.