But why did he said that he would warp him faster next time he touches Minato? Minato even stated if one of them touched first, then that one wins, AND by that he meant if Obito managed to touch Minato, he would be dead.
The warping itself takes time.
Obito said that he would try to warp him as soon as he had touched him the next time but in all honesty, it makes little difference and is mostly just arrogance.
Time and time again we've seen the swirling indicating there's a time consuming process involved, if it were instant, we wouldn't see the swirling warping for so long.
Obito has to open the dimension > set the absorption in motion > absorb. FTG users just go, end of story.
FTG's simplicity is often seen as it's drawback, because it doesn't have the extra gimmicks of Kamui but when the former faces the latter, it's simplicity gives it the advantage.