Yes, I know this issue from both sides. I tank a lot, more than anything else in this game. I always go at a very fast pace and am often pulling the next group by the time my dps is about to finish one (I always run with healers that know I'm going to do this, I will slow down if needed for pugs.).
Sometimes I still get dps though that seem to want to lead, or that focus the completely wrong mob (not really a problem in FPs but can be an issue in operations). It's annoying but whatever, as long as they aren't dying and the healer isn't complaining I just do my best to keep the mobs controlled.
Now, when I'm dpsing, I'm sorry but it is terrible to get one of those tanks that stops to survey each pull for a decade. It literally makes a flashpoint take 2-3 times longer than it should. So I will get a bit impatient when that happens and start pulling, to force the progress.
Honestly, I just don't pug a lot anymore.
Yeah, it happens that tanks are inexperienced, hell, I was once inexperienced, but it's simply irritating when they stay there and stare at the pack... so I initiate and most of the times it's sufficient for the tank to move. The other of this is when people as so hasty/don't know correct shortcuts that they pull half of the mobs from the FP in our neck.