You can't be faster than instant. This chapter proved it again, but you still has to possess the mindstate to be capable to trigger the hiraishin teleportation in time in this quick situation
Gai wasn't in on the plan. Minato even told him not care whatever happen and just go full force, yet Minato managed to appear, take the black spheres and disappear again before 8gated Gai touches him, so actually Minato displayed the capacity to teleport faster than 8 Gated Gai can bodily move.
The issue with the Jubi Jins however is that he was actually trying to attack rather than dodging/teleporting with Hiraishin. Hiraishin is instant, but to execute the senpou rasengan for example after teleportation, it depends on his raw physical speed which eventhough is incredibly fast, it's still not on Jubi Jins level especially with their godlike sensing. For a speedster specialist like Minato, surely he never(rarely) was in a situation where he was slower than his opponent in life, so this kind of situation against Jubi Jins is most likely new to him
