I absolutely agree with you, removing 5 seconds from your Astral phase is a significant DPS increase, but Fire's only cost 2.5 to cast. If you cast 5 fires, that's 12.5 seconds; you'd need enough spellspeed for 2 second casts to get it down to 10 seconds. How much spellspeed does it take to cast 20% faster, if you can't get that high, what you have is wasted.