I think people should study Maths. FTG isn't speed. Its teleportation. Speed = time divided by distance, wheres FTG is instant, therefore, it has no speed nor time limit either. So by this, we can conclude that the jutsu doesn't use speed, because its instant teleportation.
Shunshin no jutsu has been described as technique with fast movements. Therefore, it uses speed. Shisui was known by everyone by this jutsu, wheres Minato was only known for his teleportation.
Shisui is faster in foot, thus he has better shunshin.