Board Thread:Theories and Speculation/@comment-31338621-20200901181802/@comment-31338621-20200915024514

Ninja Of War wrote:

Which is why I'm skeptical of Itachi taking action and sustaining Tsukuyomi simultaneously. If we assume greater complexity, then we also have to assume greater focus required.

The complexity of a genjutsu may be tied to chakra cost, rather than concentration.