It didnt. Itachi threw a shuriken after he cast the gen jutsu
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Yet, Hachibi woke Bee up quick enough to block the attack.
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Note that despiting holding samehada, Bee pulled the swords on his back to block the attack. That means he had time to assess the attack and conclude that
1. He didn't need samehada
2.He has enough time to pull the swords from his back to block the attack
Remember the conversations between a tailed beasts and the jinchuriki only take fractions of a second in real life.
I'll take less than a second for a shuriken to reach its target from a few feet away, even if it was thrown by a normal human being.
Itachi is a ninja, with superior physical powers to your average joe. Unless he has the strenght of a granny, that shuriken would have hit Bee before he even realises whats going on. End of discussion
Inside a Jinchuriki, time isn't flowing at the same rate as in the normal world. In 1second outside, they have time to have a little conference between their Bijus and can share everything together including pain. The Biju will always know and snap him out of every genjutsu(including tsukuyomi)