Is FTG really instant?

[Lasta]

FTG in itself is instant, whatever happens b4 or after depends solely on the speed of the user

 

[Gold Lightning]

I was more thinking of his stance, with his arm behind him so he would have to swing it more, but now you mention it, WHy didn't he teleport behind Madara?

Impossible. He can teleport around the kunai. So he could have appeared in front of the kunai, to the left of the kunai, to the right of the kunai, or above the kunai. If the kunai was in the air, he could have teleported below it (Minato chose to teleport behind the kunai).

The only way he could teleport behind Madara is by throwing the kunai behind Madara or if madaras body was marked itself. Also the stance wasn't really the problem, I recon it's more to do with having one arm and having to put all your weight into that side to balance yourself.