In Google Anlaytics for App, App Speed is not the default report (In Web Property, Pag load time is an default report). Mobile App Speed Tracking needs to be configured
The App Speed reports require additional set up in your app tracking code that should be completed by the developer. Visit our Developer Guides for detailed instructions on setting up User Timing for Android and User Timing for iOS. No data appears in these reports until you define in your app tracking code which specific actions and load times to track.
User Timing for Android -https://developers.google.com/analytics/devguides/collection/android/v2/usertimings
User Timing for IOS – https://developers.google.com/analytics/devguides/collection/ios/v2/usertimings