A dedicated sprint to understand the usability issues and find suitable solutions by involving the whole team.
When you build a product there aren't certain answers, but hypotheses that you have to validate with the users.
At each significant software release, we do a validating sprint, in order to measure and validate the released value at terms of usefulness, usability and delightfulness for the end users.
Finding the right metrics to measure product values.
Identifying rapidly issues and solutions.
Improving product performances.
Improving the user experience.