You have tanks that will go from one fight to the next to the next to the next.
You have healers and dps(much more frequent) that will pull mobs for the tanks when they feel the tank is not going fast enough.
There may be a good reason why the tank is going slower. It may be a new tank, she or he may be waiting for a move, may have noticed the healer is low on mana.
The healer might need to drink to restore her or his mana.
i am a healer born, it's natural to me to heal. i have been asked by tanks if i minded them pulling big and i have said pull as big as you want but when you drain me you have to let me drink. Most often they do not ask.
i have seen tanks and dps pulling the entire room, somehow i manage to keep them up, and then they run to the boss, not giving me time to refresh my mana, not seeing my macro. it says that i am taking a drink.
i have seen tanks and dps go to room after room after room pulling everything. Then when i am drained they ignore my macro that i need a drink.
Then they die and demand to know where i was. Why was i not healing them. Call me a fail healer, that i should be able to keep up, even with not drinking, etc. Then they say something that bewilders me. They say they do not need me. They can do it on their own, i am just a fail healer.
They do not need me? They cannot handle one room pull, one boss fight without me the healer. Yet they say they do not need me. And then it happens the next time i am forced to stop for a mana break. Because they did not change their MO.
And they take no notice of people saying they need a quest. When i did Gnomeregan i needed to go to the safe area where you cleaned those Grime-Encrusted Objects. i was after the ring for the quest. So i said at the beginning i needed to go there. When we got to the final hallway i said i needed to go, that i had told them at the beginning. i said it till halfway down to the final boss, then i just turned around and went back to get there. it took everything i had to kill the monsters on my own, knocking them back, making use of all my healing and fighting. They died on the boss and asked why i was not with them. i told them that i said that i was going to the safe area, that i had said it before i left them at least several times.
You need to slow down when you have someone who needs to pause or do things slowly or is new to it or needs a quest. It also helps if you actually read the instance chat as well.