@Draegod,
Haku couldn't keep up with Sasuke's speed and taijutsu skills in these following panels:
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You can't just say that Haku was going easy on them, all he did was not kill them. To him, following Zabuza's orders
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than anything else and he was going to incapacitate them and put them in a death like trance. Both Naruto and Sasuke fought Haku
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as well.
Anyway about the cold weather, current Sakura wears a flak jacket so she'll be fine. Also Baki managed to tank a sword strike from Gecko Hayate with a flak jacket, so Sakura can use it as a form of protection to save her vitals from a bunch of ice needles.
Haku may have better speed but saying it is above part one Kakashi is wrong unless you think Kakuzu is faster than Kakashi because Kakuzu intercepted Kakashi when Kakashi was going to attack Hidan.
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The data book gives Kakashi a 4.5 in speed yet Kakuzu only has a 4.0 so even in one instance Kakashi was slower than his adversary doesn't mean he is always slower than them. As a matter of fact, using shunshin uses chakra and Kakashi was using less of it to conserve it. Haku was about to sacrifice himself so he used a maximum amount.
Anyway even if Haku is faster, which he is, Sakura has better taijutsu than him and area of effect attacks which can intercept Haku as well as destroy his mirrors. Lastly, don't underestimate Sakura's speed. She was able to send a Juubi clone flying while, then accelerated herself to a speed even faster than what the Juubi clone was travelling at and then managed to close the distance and attack it.