A Load Runner developer designs, develops, and executes performance test scripts using HP LoadRunner to measure system performance and identify bottlenecks. They simulate user activity by generating loads from several virtual users and monitor system responses, ensuring it can handle high traffic or data load without compromising functionality or speed. They analyze test results, identify performance issues, and make recommendations for improvements. They collaborate with development teams to optimize performance, and ensure the application's scalability and reliability. They also maintain and update the LoadRunner tool, ensuring it's up-to-date with current testing needs.