Sasuke is fast but not faster than Naruto. Madara never stated Sasuke was the fastest person he faced during that encounter.
When Sasuke attacked and hit Madara in chapter 674 that was a strategy that focused on the element of surprise more than a display of speed. Sasuke used a very common strategy where the user attacks their opponent from their blind spot while they are distracted. Madara was focused on implanting Obito's eye and using his s/t jutsu. It's a very common strategy that has been used time and time again in the manga. Naruto and Kakashi used it against Kakuzu.
In chapter 676 Sasuke didn't blitz Madara.
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Sasuke wasn't even fast enough to evade the shadow madara attack or even complete his own attack. He was forced to block.
Naruto was fast enough to rescue Sakura and follow through with his attack, countering Shadow Madara's attack. Naruto started his attack after Sasuke.