Mobile app performance optimization encompasses strategies and techniques for improving startup time, memory efficiency, battery consumption, network usage, and UI responsiveness across iOS and Android platforms. Performance directly impacts user retention—53% of users abandon apps that take longer than 3 seconds to load—making optimization essential for app success. The key mental model is to understand that mobile devices have constrained resources (CPU, memory, battery) compared to desktop systems, requiring developers to prioritize critical paths, defer non-essential work, and aggressively optimize the most common user flows rather than attempting to optimize everything equally.