While I understand what OP is saying I have to remind you that doing a fast run in a dungeon is not toxic behaviour. Letting someone die on purpose because reasons IS toxic behaviour. If you can't keep up with the healing because of large pulls you can say it in your party chat and let the tank know. Before you do that I strongly advise you to just try to keep up. This will teach you your class faster. Hard situations always offer better training than staying back and heal a tank who is attacking the monsters one by one.