I noticed that Holy Word: Serenity's 25% Critical Strike Chance increase lasts 6 seconds. Immediately after casting Holy Word: Serenity you will have a 1.5 second global cooldown assuming you have no haste. This leaves you with 4.5 seconds of the buff: only slightly not enough for two (Greater) Heals, with a cast time of 2.5 seconds unbuffed. However, with haste applied, we would be able to fit in a second (Greater) Heal. Some calculations give us the exact number:
BuffDuration=2(HealCastTime/(1+(Haste%/100))+GlobalCoolDown/(1+(Haste%/100)
We enter what we know:
6=2(2.5/(1+(Haste%/100))+1.5/(1+(Haste%/100))
This solves to:
Haste%=8.333...
Meaning that with 8.34% Haste we would be able to fit in two (Greater) Heals during each Holy Word: Serenity Critical Strike Chance buff. I have two questions.
1) Is this really how it works or would latency cause the buff to go away before you can get your second cast off?
2) Is it worth getting 8.34% haste for this?