Board Thread:Naruto Discussions/@comment-25977287-20150704114842/@comment-734582-20150704165952

JOA20 wrote: Minato himself stated that due to his inexperience in senjutsu he can't use it effectively. He can't hold it for long either, which is why he probably gathers a small amount, uses sage abilities then reverts to human mode. Unlike Naruto who gathers a large amount then uses numerous powerful jutsu.

So i'd say he can enter it instantly because he doesn't gather much Natural energy.