With the growing popularity of mobile apps, their complexity and resource demands have increased as end users seek more sophisticated features. This rise in complexity often leads to performance issues, which have become a significant concern for developers and are now seen as critical for maintaining competitiveness in the market. In this paper, we conduct an empirical study aimed at analyzing performance issues and their causes in Android apps. To achieve this, we examine 385 Stack Overflow (SO) Q&A posts out of total 6059 posts. We qualitatively analyzed and formulated two taxonomies: one for performance issues and another for their causes (first of its kind). Our study identifies key categories of performance-related issues, including textit{UI/UX & Graphics, Development/Coding Practices, Performance Optimization, and Data/Database Handling}. Similarly, the main categories for causes of performance issues include textit{Inefficient User Interface Design, Poor Resource Management, and Lack of Development/Coding Practices} among others. These comprehensive taxonomies serve as a checklist or guide for developers to help them identifying, knowing the causes (diagnosing), and resolving performance issues in their app development.