That's not what you said, you said it doesn't build up chakra into the eye, but it does.no ,the speed of kamui differs according to the user will ,obito himself said that he would kamui faster the next time against minato
no it isnt the same thing ,if kakashi hasnt enough chakra for kamui ,he cant use it unless he regained his chakra just like the case with any jutsu in the manga ,that doesnt mean he needs to create more chakra everytime before using kamui even if he has enough chakra