As I said, FTG is instant, minato's movement after it is not. Madara with rinnegan sensing and possibily SM sensing reacted to tobirama's atack after FTG.
Minato BM has the same speed, or is a bit faster than BM naruto(without FTG, of course). BSM naruto has his speed enhanched by the senjutsu. So he might have the same speed as minato or a bit above.
As for reaction speed, he displayed enough in the battle with juubito(even minato had problems with juubito as well). His sensing skills will allow him to sense danger, and his BSM speed will allow him to dodge it.
EDIT: Concerning your last post: The kunai is a few cm far from obito. Mianto is fast, so after teleporting to it, it would barely take him a second to get to obito.
But minato's movements after FTG aren't instant. We saw madara reacting to post FTG movement.