Black zetsu infected naruto just before transfer then pushed himself between naruto and minato's palm so that he was the new target. It was done so fast because that's just how minato is, he's very good at sealing. It's the same way he initially sealed kyuubi in naruto so quickly. All he said was he's sealing now and it was done, there was no wait time.
The only real surprise here is that minato can do seals with one hand (BAMF)
And, confirmation of how fast Minato's rasengan execution is. he switched from sealing to a rasengan in an instant.
Plot. Kishi is losing it.
Most of all question ; Why do Kishi and kakashi having such an hard time dealing with black zetsu ( i know i know obito blabla plotshield) but someone from the sword mist solo'd him with 1 slash?
You're forgetting that was after a long battle with Mei, KCM naruto and others as backup. Chojuro didn't kill zetsu easily, and zetsu didn't even die.
