Thats the problem, Hiei has alot of feats and history to look back at but feitan doesnt. All he has is his fight against zazan but that fight showcased alot. He was so fast that he was leaving after images (which was at the beginning.He wasnt even at full strength when he fought her, the girl with the vacuum said he was sluggish, and the guy that controls people with the needle said he wasnt at full strength. He got his arm broke and continued to fight. Then he got pissed and transformed and used that rising sun technique. He was walking right inside of it casually while zazan was burning to death, which says he can take enormous levels of heat somehow. He said and i quote "Youre done already ? The heat wasnt even that bad. you could've had a quicker death if you wouldnt of dealt me more damage." Which leads me to believe he might have another form or a crazier technique, or he just wasnt trying i dont know. Also we dont get to see what his speed is once he transformed.