Long range Kamui on a small object (like the head of a person) works in the same window of time of Obito's intangibility (
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) and of a movement of 5 cm from KCM Naruto (
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). Its activation speed + jutsu speed is close to instantaneous, and Kakashi's reactions and timing, and precision too, are outstanding with his feats. That said, you're right, it still isn't instantaneous as fast as it can be, when FTG is. But FTG needs to be activated, it works only if the user can react in time.
Can Minato react to a Kamui snipe after a diversion or a strategy (with things like Raiton Kage Bunshin, smoke bombs, Suiton + Raiton combo, Doton etc) by a master of tactics and deception like Kakashi, who knows the Hiraishin no jutsu and its mechanics? When Minato barely evaded Obito's short range Kamui, that is not as fast and not as unpredictable as Kakashi's long range Kamui? When Kakashi can do snipe feints to trick him into warp to an already noted kunai and then use a full speed Kamui warp to snipe him before he can react (as Kakashi did to fool Obito: he feinted to warp away a Rasengan, then warped the clone's body at full speed and at the perfect time to fool Obito in believing he hit him (
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)(
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), this inspite Obito having full knowledge (
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) and reflexes at the very least comparable to Minato, he still wasn't able to react to that Kamui (
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), that was so fast to warp the full body before a Kamui-propelled projectile could hit him from point blank)? When even Minato himself couldn't perceive what happened when Kakashi used Kamui on the Gedo Mazo while Madara was summoning it? I don't think he can.
Kakashi is also far more complete and versatile than Minato overall. They are both fast and smart, but Minato is just Hiraishin if he has to fight high level opponents, maybe fuinjutsu and Gamabunta, if we want to stretch; Kakashi has Raiton mastery, clones mastery, top notch ninjutsu, taijutsu and genjutsu skills, and Kamui as his trump card. Minato has Hiraishin but to land a hit he necessarily needs to outperform Kakashi and touch him or land a seal on him; it's a lot more reliable to think that Kakashi would manage to make him touch a Raiton Kage Bunshin or to time his Kamui to warp away his head, with the chakra to use at least 6-7 Kamui shots. Gai has also a flawless teamwork with Kakashi, Tobirama and Minato are a great team but not on that level. Gai heavily pressures them and Kakashi is ready to snipe.
Gai has proved to be faster in combat speed, reaction speed and movement speed that Minato and Tobirama can using Hiraishin with their level of reactions. Hiru Tora can neg them both, as its too fast for them to react and especially its AoE is too big to save themselves with a Hiraishin mark on the field, while Gai can shot it without being afraid to hit Kakashi, that can warp in the Kamui dimension and then warp back.
