Choosing the "most important" performance metrics for your app depends largely on its type and functionality. But tools like HeadSpin can shed light on various metrics, helping you identify what truly matters most. Here are some general contenders, but remember to prioritize based on your specific app:

User-Focused Metrics

Resource Utilization Metrics

Business-Aligned Metrics

Remember, tools like HeadSpin and LoadRunner can capture many of these metrics and help you analyze them to identify what truly affects your app's performance and user experience. By focusing on the metrics that matter most, you can optimize your app's performance and deliver a stellar user experience!

You can read about it more in this blog on performance testing tool.