How can you call AST slow compared to WHM? They have Lightspeed, and instant GCD heals.
WHM is by far the slowest healer.

It's true that AST's weakness is mana regen, but... every bad healer I've encountered has been a WHM. Mainly because all they do is spam stone, because they think they can do that and get away with it.
ASTs are a support class through-and-through.
WHMs are DPS who occasionally go "oh shi-" and then start healing.