A Java Performance Developer is responsible for optimizing the performance of Java applications. They perform tasks such as code profiling, memory management, and garbage collection to improve application responsiveness and speed. They also identify bottlenecks in the system by using performance monitoring tools and make necessary changes to enhance efficiency. They test the application under different loads and stress scenarios to ensure its robustness. Their role also involves collaborating with other developers to implement best practices for Java development and to optimize the overall system performance. They continuously monitor the system's performance and propose enhancements when needed.