Aellas, please stop spreading misinformation based on your LFR and low key logs.
Every holy priest and every holy priest guide just screams that mastery is our worst stat for M+ dungeons.
If you want to cast every spell you have, pump prayer of healing, run the divine image legendary, and stack mastery thats fine. But the fact is that its not the way to go. You need haste crit vers more in higher m+ keys.
Lets just stop replying to this guy, and let him live in his bubble. He does not take any advice at all.