Fast hands are usually just how you’d perceive short punches, hips before the hands - Joe Louis and an 80s Tyson come to mind. Jimmy Young also was really speedy and a good body puncher too. I remember seeing a clip a long time ago of James J Corbett KOing someone sparring he was so quick it stayed with me - I can’t testify to the quality of the video or if it was adjusted for real time however.
If we're going by overall body (twitch fiber), not just one attribute, it has to be Patterson no? I've always seen him as faster than Ali and Tyson, maybe not as well of a mover as Ali, but just speed..........yeah