What makes a class needing Crit or Det or D.Hit in priority? (mainly about MCH)

[EllieShadeflare]

That really doesn't matter, because even if you had so much more spell speed that the Fire IV cast time was absurdly fast like 1.5s, it would still be longer than the GCD.

In a sense, you're right, but I was more nudging toward Fire IV's relationship to Astral Fire (or rather, its inability to refresh that). Mostly, Fire IV's design isn't that well balanced with the new Enochian mechanic as it is, and only Spell Speed can solve that to any extent.