There are some times where we do some tricky things under the hood to make things feel better, and this is one of them. The intended gameplay is that you should be able to get a Heating Up proc while already casting another spell, and follow that one with an Inferno Blast to force a Hot Streak proc. You should never need to cancel your current cast to get that benefit.
So, tricky under-the-hood things follow. Again, you do not need to know or understand this to use Inferno Blast effectively, or play Fire; it should “just work." But, for those interested in the gory details, here they are:
-- Fireball, Pyroblast, and Frostfire Bolt (which was bugged until recently) have a cap on their effective travel time.
-- Their effects will take place after a maximum of 0.75 sec.
-- The fireball visual will still travel at the same speed, but you’ll see the damage number pop up and get the Heating Up proc after 0.75 sec, even though the fireball visual is still traveling through the air.
-- We normally prefer the game to feel more visceral, by tightly syncing visuals to effects, but this change felt worth the gameplay benefit to us. In a future patch, we may be able to hasten the visual to match.