It's because you gotta think Sasuke is in base mode and naruto is in RSM. That's another thing people took note of with his speed. He was running as fast as Naruto in RSM, which KCM was faster than raikage, who was the fastest ninja behind and after Minato('s death) so basically Sasuke in base mode is as fast and physically strong as RSM naruto.Except for one thing sasuke was always beast in taijutsu. And holy banana why do ya keep bringing up keeping up with naruto as if sasuke was weak. U do have a point though it does kishi forget the type of things naruto has or something dude has not shown his FROG KATa after pain its always fire power rasenshuriken type of stuff