Man, don't get angry at me, but can I tell you a big secret about Silent Killing and all that Mist stuff?
First of all, he isn't moving any faster than usually, it's just the others are moving slower. Get it? And why? Do experiment yourself. Get in the dark room for couple of days. Where everything is dark and there is just a little bit of light. Now, when the cuople of days past, invite someone over. And, look at a wonder, what happens next?? You've been there for a few days and your eyes have adapted to the environment. Normally you'd move better than your friend. THAT'S THE BIG SECRET. Zabuza is adapted to mist, while others are not. Because of that it seems he's 'moving fast'. That's not true. Others are just moving slow. But imagine SM Jman, Naruto, Minato, Kabuto and any other powerful sensor type? The stomp Zabuza with no difficulty.
And you really think Kakashi couldn't kill Zabuza one on one? You are amazing! Help from Naruto and Sasuke? That's called plot. They had to fight him to get the sense what shinobi life is really about.

If they met one on one, Kakashi would stomp.
Also he survived because Haku helped him. How is that any better than Sasuke&Naruto help?
Just deal with it. Zabuza is a fodder. It will hurt you less.