Is this only the case for the rapid fire? If it is, then I think it's because RF is a channeling spell, so when you start it, you are set in channeling state, and only shoot soon after that, and it's more like a delay in game mechanic, rather than lag.
This is a case where they want the animation to be as fluid as possible, but the gameplay is the bottom line. So, the responsiveness of shooting is close to immediate and the animation is slightly behind. When you're hackin' and slashin' it's hardly noticable.
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