Assuming you are maintaining a high velocity but your customer isn't keeping the pace with the team in-terms of feedback and freezing the stories. what do you do? Do you decrease your velocity or keep the pace and develop things based on your assumptions?My response:
Talk to your customer. If they are happy with the pace, then you could downsize the team to match the customer velocity and use those resources elsewhere.Your thoughts?
If they would love to go faster, then work with them. Learn how to help them be a better customer, or immerse some of your team in the activities of the customer (slowing your team down and speeding them up).
View the customer as being part of the team, at least until you can solve the problem.
The goal is not to develop stuff fast... the goal is to deliver the right stuff fast. (therefore developing things based on assumptions won't help).
Another thing you can do with that "extra time" is reduce technical debt, train the team on new stuff, refactor code, and improve things like performance and installability.