When the quickroute track is shorter than your actual run (e.g., when you make sharp turns around obstacles) doesn't that explain the slower speed? Can you make your GPS record you position more frequently?
 

I have the watch set to record at 1 second intervals. I don't think I'm making sharp enough turns that it is screwing up the speed measure in QR. I think the areas where I slowed down are pretty accurate. I think I just got lazy on the way to 6. I also took a good look at the route choice from 7 to 8 (planning ahead) and decided that using the road to the left of the straight line was fastest, but I didn't take it because it didn't seem like much fun).

Michael