Haku was matched in speed by sasuke, when sasuke used the sharingan, rock lee in normal form beat sasuke and sharingan, because of his speed, Neji defeats rock lee even with his speed.
The byakugan also gives neji and advantage with the vastly superior feild of view, and would enable neji to see which image of haku was real, and not just a mirror image. He also has his spin defense that would deflect all incoming attacks. Combine that with the chakra strikes, Haku is pretty much screwed.
"No no, I think that once he had captured Neji in the Ice Mirrors, I dont know how long he would be able to rotate around to stop the needles, as Haku was way too fast for Sasuke then, and I dont think he;d be fast enough.Sasuke with the sharingan and after being worn down was able to see haku and detect and deflect vital attacks. He actually lost because he decided to protect Naruto. A fresh Sasuke after that incident was completely demolished by rock lee's speed while still in weights. Neji at that point always beat rock lee. Neji is faster, stronger, smarter and has way more in his aresenal than the sasuke that haku fought, and the Rock Lee that Sasuke was subsequently embaressed by. This goes to Neji.