See at about 1m00s here, they're only doing about 15mph right at the top of the banking, it's not like you have to be doing 30mph to stay up.
Yes, I'm not saying you have to be going top speed but at HH training sessions people ride up to the top when they're too tired to stay in a group/after a sprint and need to rest so are both decelerating and too knackered to keep pushing the pedals hard. You can comfortably crawl around the top at less than 10mph at HH, whereas I don't think you could do that at Stratford without slipping to your doom.
Yes, I'm not saying you have to be going top speed but at HH training sessions people ride up to the top when they're too tired to stay in a group/after a sprint and need to rest so are both decelerating and too knackered to keep pushing the pedals hard. You can comfortably crawl around the top at less than 10mph at HH, whereas I don't think you could do that at Stratford without slipping to your doom.