Flame Blast drains mana based on cast speed - bug?

While testing different staffs with Flame Blast, I noticed that the "Mana per second" for Flame Blast in the tooltips is not necessarily the same amount of mana you drain when holding down the Flame Blast button. It actually scales with casting speed. It might be intentional because it obviously blasts faster, but its very punishing if you hold it too long (holding it for more than 1 second is punishing for me, I invested a lot in casting speed without noticing it earlier) so I personally dont like it. If it is intended, it should at least be hinted at in the tooltips.


Maybe other channeling spells have the same problems?


// Otherwise, just info for other players who might be wondering.



Edit: Flame Blast on yourself or the orb does not change the mana drain.