The 8 minutes wouldn't exist without the 20 minutes of unreasonable waiting Luffy did to attack. How do you explain Luffy punching through Doflamgino's ultimate technique easily? It's because he was too much for him. Because when there was nothing left in the war to talk about, Oda had to finally be realistic about the fight and draw Luffy attacking him without messing around.
why is it hard to just acknowledge that Luffy coudn't beat DD in 20 minutes? Simply as that, and yes, Luffy's final attack was stronger that Doffy's 16 strings, he even went above him (that's a mirroring how he beat Crocodile), so what, it was obvious that Luffy is going to win, same with Naruto and Ichigo and etc, they all are MC. As much as I love Luffy his ass was pulled through this arc waaaaay to long, and his fight with DD showed that he is far from the top of the world (but now he can be on par with many of high levels of new world).