Where I live there are no nearby flat water spots ... however, I have windsurfed & kited at many different locations. I have been going to Cape Hatteras for 35 years. Back in the windsurfing days I would go as fast as I could in Pamlico Sound - I even had a speed needle that I used to ride. It was fun. Now I don't have much interest in riding as fast as I can ... on a kiteboard, even in very flat water. And I don't see that it is much of an interest for most kiteboarders in Hatteras. Boosting or freestyle in the slicks, yes. My interest is riding oceanside in the waves wherever possible.
This is not to say that kiting fast isn't a thing for specialists such as yourself, but I don't see many ... actually, ANY kiters that I know doing it. Even foiling, which started as more of a speed/racing oriented thing has become much more focused on slower foils for carving & wave riding.
As far as the OP is concerned, the answer to the question do windsurfers, on average, in average choppy conditions, go faster than kitesurfers ... is yes ... assuming that the windsurfer is adequately powered. However, overall kitesurfers are able to maintain a much more consistent speed due to the ability to create additional apparent wind. Windsurfing fast in average conditions is (IMO) relatively more fun on a windsurfer than a kiteboard. Which is more fun or faster in speed specific conditions is a different question, which I can't answer, as I've never ridden a kite speed board in flat water.