Originally Posted by
Achi3
Won't work as you said. The cast time is calculated when you actually start casting a spell, so for example if you manage to activate icy veins (or get a bloodlust) during the cast, it won't increase the casting speed of your current spell, it will only be applied on the next one. Same goes for an effect on next hit, like the icy veins "proc", it will only work on the next spell.
Edit: to illustrate it better, it's the same as applying a curse of shadow on a mob right before your shadowbolt hits it, your shadowbolt won't take the curse into account and will do normal damage.