Forget my common sense man, you need better eyes. I think you're just blind:
1- BD is not a fast technique at all. Heck, Minato summoned Bunta and it was still charging in ninetails mouth whilst he sat on it. This is manga fact, not your crappy posts. Naruto blitzing it is a laughable point. If you want to truly talk about a speed feat, then bring out his outmaneuvering A. But this? Like shit son, this is lame.
Not only that, he's covered with Kurama's Chakra as well. It's a shroud actually that improves further with BM. We are all well-aware that even at half its power, Kurama is significantly stronger than Hachibi and with a better shroud in BM, he even took Juubi's flash dama head on albeit it's not at its full power. What a feat. He blitzed something that is slow and has a shroud, but omfg, he deflected something.
2 - Because he's using Kurama's chakra? Seriously, you are still in the dark about this? With Kurama's chakra, he can even summon larger toads, was able to create Rasangan with a single hand on a few occasions, and even gave actual shape to FRS during his fight with Kakuzu. He cannot even make them in this manner in SM. Kinda proves your fanfiction wrong, doesn't it?
Yeah, because Itachi was fighting KCM Naruto only? My bad. You are totally right.