Yes, thats basically what you people are saying by stating "tsukuyomi is instant", the casting may be instant but the events that occur within the illusion isn't.
The genjutsu is instantaneous real time. The illusions the victim suffers last as long as the user wants. I don't know how to make this any clearer.
Let me try. Its like when you go to sleep at night and you have a dream. In your dream, time may go very slow, or very fast. People have different dreams, but lets assume this dream happens very fast. You go to sleep at midnight. Your dream, lets say lasts [in the dreams time] about 5 minutes. You wake up. Its not 3 am.
Same principle, just you know, the other way around.
1. Itachi traps you in Tsuuyomi [the casting is instant].
2. Your
mind is trapped in this illusionary world. Like a dream, only one that you cannot voluntarily escape.
3. In the real world, time continues normally, with your now unconscious body at the mercy of Itachi. But why would he kill you when he can torture you.
4. Itachi [in the dream world] is able to manipulate space and time, so that, even though time is elapsing differently in the real world, much like a dream would be, the illusionary world's time moves differently, at Itachi's discretion. Itachi can make 1 minute of real time 1 year in the illusionary world. He has complete flexibility.