Handspeed is among the most important qualities for a fighter. If you know nothing about two fighters who meet each other, you usually get your prediction correct by picking the faster guy. However, if a fighter bases his fighting on speed, he is in trouble if he meets even a faster guy. Chris Byrd and Eddie Chambers failed miserably as they tried to go down in weight and faced faster guys. Joshua is usually faster than his opponents and lost against quicker guys (Ruiz wasn't better that Parker, but he was faster). Even Ali was troubled by guys who could somewhat keep up with him. Jab is similar. The best of jabbers, such as Holmes and Lewis, had difficulties against fighters with good jabs, even if they weren't as good otherwise. Usually trying to beat a fighter in his own game isn't a good idea, don't hook with a hooker is sound advice. However, if that game is either speed or jab, then it is worth a try.
Chris Byrd failed because he was old and basically a skeleton. His handspeed didn't win him any friends at 200+ but it wasn't blazing either. Corrie Sanders had troubling speed. He sprang traps that snared guys who knew how to box. Chizz thought he was going to be the faster man. Sanders also obviously displayed "troubling speed" for Rahman, Vitali, Wlad and others. Calzaghe was all about the speed cos he had no major power. Speed, up and down. Judah is a great example as he had no brains but had a borderline HOF career nonetheless. Amir Khan likewise. He beat "Better" guys by basically being way faster. Take away Amir Khans handspeed and I see him struggle vs Michael Gomez.
Was just re-reading my post thinking that I should change it from "struggle" to "lose". Gomez really was a talented guy who should have made a lot more noise than he did.
Byrd was kinda fast but not that fast really, nothing special anyway, he was just awkward. Sanders had legitimate speed though.