Sort of an odd question, since I think you'd only get the answer if you picked one up out of context and placed them in a race to see how they performed. Maybe a courier who also trains will come and chime in, but I suspect it to be more about acceleration than average pace, since the job requires darting in and out of traffic, to and from stops and generally around obstacles. An average of that sort of riding might not be a terribly telling stat. I'm sure they're quite fit on average and are superior bike handlers.