Attention AST: Lightspeed is one of your better spells, not a bad spell.
There was a thread here the other day someone posted mentioning that Lightspeed is practically useless. I want to clarify that Lightspeed is NOT a bad spell. That spell can single-handedly spell the difference between a wipe or survival. Sure it doesn't see use too often, but when it does it is brilliant.
I just got done doing Sohm Al for my first time, and in one part during end boss the SMN in the group was insta-killed, aka my skill as a healer was not the reason they died. Other than that everything went smooth. So I cast Swiftcast and Ascend to bring them back. But because I used the Swiftcast, I had no fast way to heal them up and with the frequency and potency of incoming damage I was afraid they would die before I could heal them as the burst heal Essential Dignity was on cooldown. At that moment I remembered Lightspeed and instantly cast it and then followed up with an instant Benefic II.
Basically I think a great goal for AST is to balance use of Swiftcast, Essential Dignity, and Lightspeed as best as possible so you always have a way to instantly heal. Basically I think Lightspeed is a good last resort for instant healing if Essential Dignity is down and the situation is dire enough for it, such as topping someone up after revival. The 25% reduction in potency is not an issue here because the goal is to get their health high enough after ressing to survive the next hit.
The reason I said mean the difference between a wipe or not is not because we almost wiped, as we didn't. I was just thinking of situations where it could be.