The best way for the Scrum Master to assist the Product Owner in this situation is to coach them on how to effectively communicate the client's concerns to the Development Team and encourage the addition of the performance issue to the Product Backlog. This approach ensures that the Development Team is aware of the issue and can prioritize fixing it in their upcoming Sprints. It aligns with Scrum principles of transparency, inspection, and adaptation. Telling the Product Owner that performance is defined by the Development Team or noting the issue for the Sprint Retrospective might not address the urgency of the client's concern. Notifying another team does not align with the Scrum framework principles, which emphasize the Development Team's responsibility for all aspects of the product being developed.