Speed and striking speed are two different things. Minato has more efficient FTG and better Shunshin so he's FASTER, but the manga already stipulated his striking speed is slower. Tobirama marked Obito as a Jin multiple times when Minato and him were fighting with Naruto and Sasuke against him, whereas Minato wasn't able to. Likewise, with Minato's reflexes enhanced via Sage Mode, he still wasn't able to land a hit on Madara who reacted to his STRIKING speed. Tobirama failed yes, but that's against someone with the second best Senjutsu next to Rikudou Senjutsu (using Hashirama's Sage Mode). We aren't talking about KCM or BM Minato who are clearly on another tier. This is a theoretical fusion of the alive forms and taking their best traits.
For The First Part...
Tobirama Reacted To Juubito & Tagged Him Multiple Times Is BS!
The Explosive Tags Attached To Him When He Ripped Half Of Tobirama's Body & Passed Through It,As Its Impossible To Attach Them With Hand Due To Their Position One Was On Obito's Leg
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...Did Tobirama Crouch To Attach That One?
Tobirama Tagged Him 1 Time & Even That Was After He Got Hit.
The FTG Mark Was On Juubito's Right Armpit As You Can See
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,So According To Their Position
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I Believe Tobirama Tagged Him With His Left Hand After The Right Side Of His Body Was Ripped Off.
& For The Second Part...
For God's Sake Don't Compare JJ Madara To SM Madara U_U
Minato Faced Stronger & Faster Opponents Each Time So There's No Comparison.