I think that how fast someone rides on the street is determined by a combination of rider skill, how close to the edge he is willing to push it (90%, 70%, etc), how well he knows the roads, and how much he is willing to exceed the posted limits. As for the bike itself, how comfortable the rider is with the bike is probably more important than the outright limits of the bikes performance. There are always people discussing which bike "should" be faster when it is often irrelevant on most public roads.
I am talking about bikes with reasonably high performance limits here. Obviously there are some bikes which will limit speed even on public roads if they have limited cornering clearance or very limited power.
I ride, Therefore I Am.