I struggled up a steep short climb that had a 20% pitch at the end. It was the hardest climb I've ever done and when I got to the top and got off my bike I collapsed and was lying on the ground gasping for air. I don't know what my heart rate was but I didn't have another tiny bit of effort available. I can't conceive of the physical shape it takes to actually race up an even steeper hill. I'm sure I was below 4mph on that grade. Coulda walked faster.