Karthus does not cast spells as quickly in his passive form as he does in his normal form

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I saw a thread from a few months ago that mentioned this. I have only been playing the game for a few months and I feel as though I noticed this bug immediately when I began playing him. Karthus was one of the first champions I purchased and he is incredibly fun to play. However, it seems as though there is a delay when he casts his spells in his passive form.

Clarity: When you are playing Karthus and you start casting spells when your passive takes effect you do not cast them as quickly as you would when you are alive. Particularly with "Q", there is a delay.

Bugs are bugs.