Application performance issues can be caused by various factors such as network latency, slow servers, database problems, code quality, misconfigurations, and third-party dependencies. To address these issues, developers should optimize API requests, use asynchronous communication, monitor and trace server-side issues, implement query optimization, improve code quality through regular refactoring and testing, configure resources appropriately, regularly audit third-party libraries, and isolate third-party calls using background workers or async processing. Using a comprehensive debuggability platform like Sentry can help developers efficiently discover, trace, and resolve performance problems in their applications.