iOS app performance monitoring is crucial for enhancing user experience, as poor performance can lead to high uninstall rates and negative reviews. Key metrics for monitoring include app stability, launch time, network response time, logic execution speed, and UI response time. Tools like Luciq APM provide real-time monitoring and detailed insights into crashes, network issues, and performance bottlenecks, helping developers address problems proactively. By establishing target thresholds and using metrics like the Apdex score, developers can gain a comprehensive view of an app's performance, aligning team efforts and improving app quality before negative feedback arises.