Boredom

I think you are overlooking my main point. You can have a heal take 3 hours of real time because you think it's "balanced" or w/e, but that will just dissuade people from playing the class or taking that spell. I'm saying if many priest spells take multiple rounds, people won't want to play a priest because it's boring; unless they want to write a paper while they play their character ("I start my heal, wake me up in 20 minutes.").