It pays to be patient with new tanks. Being impatient with them turns their dungeon experience into a miserable one. This increases the likelihood of them not returning - which in turn means longer queues for me.
Sure if I can see that the group is handling the mobs with no problems, and the tank is being overly cautious I may try to encourage him/her to speed up a bit but it's all about how say it:
Saying "Looks like you are doing great atm. If you feel ok with it, I am confident you'll be able cope with pulling a few more mobs
" is much better than saying "comon fail tank y u so slo lol".
With the kind of gear I have these days, I suspect I can solo most of the 5 man content now as a hunter (I haven't stepped into a 5 man heroic for a while now), but really, if I'm doing LFD there is no need to create stress for the rest of the group.
At the end of the day you're either part of the problem or part of the solution. Even if a slow tank is frustrating, becoming part of the problem is not going to help anyone, and it's only going to make your own experience even worse.