Since you do a huge pull, Mana will regen before you do the next huge pull if you keep out of combat properly.
It is empirically superior to speedrun than it is to single-pull, if we're talking just pure Healer damage potency.
Not to mention the competencies of other party members' AoE.

10 is also a relatively conservative number; depending on dungeon. Obviously the benefits of speedpulling vary from dungeon to dungeon ,but it's never not good to speedpull if you're capable of it.