Speed is nothing without awareness and recognition. You need to understand what's going on around you, and anticipate what is going to happen in front of you. Otherwise your speed is irrelevant. And when you're not taking yourself out of a play, the opponent will be. You also need to be able to avoid and shed blockers.
Make no mistake, Zach still has his speed. But the reason Zach still makes an impact in this league because he prepares as well as anyone. When he sees something happen in front of him, there's no hesitation. He is full steam ahead. That's instinct and preparation.