I think it's just that Sasuke is really fast, it said so in the fight against Zabuza. Body flicker is a space time jutsu. It's like a summoning jutsu, kind of. Instead of summoning an animal instantly you're sort of summoning yourself to a place. The speed dosen't vary it's the same thing every time.
I don't know if anyone took the time to actually address this, but here are the FACTS.
Body Flicker is only High Speed Movement, nothing special
What Minato uses is a unique self-summoning jutsu that manipulates space and time. It's something even the Sharingan can't track.
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Body Flicker Technique (瞬身の術 Shunshin no Jutsu)
This jutsu is a high-speed movement technique, allowing a ninja to move short-long distances at an almost untraceable speed. To an observer, it appears as if the user has teleported. A puff of smoke is occasionally used to disguise the user's movements. It is accomplished by using chakra to temporarily boost the user's reflexes. The amount of chakra required depends on the overall distance between the user and their intended destination. Shisui Uchiha, one of the most talented Uchiha, was feared as "Shisui of the Body Flicker" (瞬身のシスイ, Shunshin no Shisui), likely referring to this technique.
The Body Flicker Technique has various variations among the different hidden villages, all of which involve some extra element aside from the movement itself. The extra element is usually some sort of substance used to distract the opponent, such as leaves or sand. Gaara, for instance, uses the Sand Body Flicker (砂瞬身, Suna-Shunshin), which uses sand to cover his movements. Other varieties include the Mist Body Flicker (霧瞬身, Kiri-Shunshin), Water Body Flicker (水瞬身, Mizu-Shunshin), and Leaf Body Flicker (木ノ葉瞬身, Konoha-Shunshin).
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"Flying Thunder God Jutsu" (飛雷神の術 Hiraishin no Jutsu)
This jutsu was created by Namikaze Minato, and quite possibly the reason behind his nickname, "The Yellow Flash of Konoha" (木ノ葉の黄色い閃光 Konoha no Kīroi Senkō). It allows the user to move over himself and/or a target he touches to another location in the blink of an eye. Although it is often compared with the Body Flicker Technique, the speed of this jutsu is much greater. In principle, this jutsu is similar to the Summoning Technique, as it revolves around manipulation of time and space rather than accelerated movement, but instead of bringing something to users, it sends them somewhere. Additionally, it makes no noise and produces no puff of smoke, allowing the user to appear behind someone unnoticed.
To activate this jutsu, the user needs a special seal or "jutsu formula" (術式 jutsu-shiki) to mark the destination. After this is done, the user enters a dimensional void that almost instantaneously transport himself to that location whenever he chooses from then on. Minato applied the formula in advance to weapons such as kunai. By giving one such kunai to a person, he can immediatly teleport to that person at any time. He can also apply the mark to a touched area, such as his opponent or some other surrounding feature.
Furthermore, it may also touch the ground and Minato knows immediately where all enemies are and can mark his goal. This technique also possesses surveillance purposes as he can continuously sense where anyone is as long as the seal is on them. No matter where an opponent is, he arrived directly to him. The jutsu is also especially special because it requires no use of chakra, do to a special secondary seal that emits the same chakra as him, allowing him to have an instant connection to the area.
This is one of the few known non-Kekkei-Genkai techniques created that the Sharingan/Byakugan cannot follow. It is because of this technique that Konoha came out victorious in the Iwa-Konoha Ninja War. After having his subordinates toss marked kunai randomly into the enemy ranks, Minato used the technique to appear within their ranks, using the great speed of this jutsu to seemingly appear to be in several places at once and wipe out entire squadrons one by one in moments before they could even react. The Japanese name Hiraishin is also the phonic of the Japanese word 避雷針, which means lightning rod.
