Name some big fighters who were quick with their hands & on their feet. One that comes to mind for me is Corrie Sanders (RIP).
Buster Mathis Sr had good speed in my opinion. Anthony Joshua without the muscle mass looks pretty fast for his size.
Any Ruiz is probably the fastest 260+ pound man that ever boxed. Two tonne Tony threw a fast hand too
Waldo and Fury and from olden days, Wills. As highly as I rate Lewis, his speed wasn't some revelation. He was a bit leaden footed at times.
I'd suggest Da Preem, but that'd be unfair, as everyone already knows he could outspeed Pep and Whitaker on his best day. In all seriousness, Tubbs, Fury and Ruiz Jr. are all good nods, and I'll throw in mentions of Greg Page and Ruslan Chagaev.