Event request: UNIT_CAST_SPEED
 

Similar to UNIT_ATTACK_SPEED, but for casters.
When my priest gains the buff from Wrath of Air Totem, the cast time return value of GetSpellInfo() is not yet updated when UNIT_AURA('player') fires. Apparently there's a gap between gaining the buff and actually getting the additional haste.
When gaining Heroism, the UNIT_ATTACK_SPEED event fires about 400ms after the UNIT_AURA('player') event. I assume there's an equal gap for spell haste changes.

Hmm... I'd be interested to know what causes this delay, and if it exists for other buffs. I don't think it can be due exclusively to the processing time of something on the server, or to the latency, but I can't think of another restriction that could cause a 400ms delay.
Is this delay there on purpose?

Is there really a 400ms delay? That seems longer than necessary. I assume it has something to do with server latency.

While I would advocate something like this, I would propose, for a temporary workaround, some magic with OnUpdate and polling GetSpellInfo(), watching for the cast time to change (maybe with a 2s timeout or so, just in case something went wrong). It wouldn't be the most efficient of algorithms, but I don't anticipate any major CPU overhead in doing this and there isn't really a better way.