Board Thread:Boruto Discussions/@comment-28432164-20160514001744/@comment-26127155-20160514233919

@Lulcielid

In this case Naruto's reflexes would not be as sharp as they usually would be, and he would not be at his peak physical condition. This may not seem like much, but it would greatly affect his ability in combat as these are the factors which allow him to be more efficient in battle, and keep him on his toes to react to any attack enemies throw his way.

When Naruto was 17 he was constantly in fights and it is safe to assume that his peak physical form would have been during that time period as they have emphasised that not much bad stuff has happened since the 4th great ninja war. His lack of training and combat would make him a shell of himself like how Hanzo was against Mifune and it would allow other characters to catch up and surpass him.