In my Nike Lunaracers I ran a 5K in 19:13. This predicts around 39:00 for a 10K. Two weeks after the 5K I ran the Bolder Boulder 10K. I've run the Bolder Boulder 15 times, always faster than 39:00. I found a pair of the Vaporflys on eBay, used once in a 15K race, for $280. This was a week before the Bolder Boulder so I didn't have time to train or try them in a 5K. I ran the Bolder Boulder in 40:00.27. My heart rate was good and I felt good but the shoes felt slow. They're a full ounce heavier than my Lunaracers. They're the stiffest shoes I've ever seen. I'm a forefoot striker, the shoes feel different at jogging pace heel striking (bouncy castle) vs. race pace forefoot striking (they lose their bounce). My guess is that these shoes don't work for forefoot strikers because that big bouncy heel is no longer doing anything.