I think the thing with hand speed is that the speed has to be realistic. What i mean by this is id measure speed as the number of punches landed in a given time, with full impact, not pretend hit and against at least a moving target but i suppose static would be reasonable.
But you have to land them fully. Its all very well doing some very fast hand movements but in reality landing a punch sucks the speed and "flow" out of your movements, slowing you right down. On top of that it has to be punches that would really hurt, not just simply making full contact with a limp wristed attack.
I dont think we really know how fast alot of people are as we dont get to see this.
Moving someones arms quickly in the air against pretend attacks takes far less time to prepare than the above.
and remember speed is only one component, slow powerful punchers can take out faster guys if there is a big power difference.
Speed does not equal power if its at the sacrifice of body mechanics, which it often is.
This guy for instance we know can punch fast in the air and irl. Although you can see the speed decrease alot when shots have to actually land, they are also slightly different skills in my opinion
http://www.youtube.com/watch?v=M5U7pdfzT4A