I know how you feel. As a healer main I often run across speed tanking uhh Tanks who don't wait for protect or anything. All I can do is grumble under my breath about how annoying I find them. I usually wait till they come to a stopping point (usually when they gather the mobs) and cast whatever I need. If I'm feeling really petty I'll let their health go dangerously low before healing them

Though I don't think we should be letting them die if we can help it as that usually causes wipes sometimes and tends to slow the dungeon down.

It's all up to rng honestly, there are some less than pleasant people you'll run into. Don't let it get you down!