For more information on debugging janky (non-smooth)
applications, see Flutter performance profiling.

Measuring app startup time

To gather detailed information about the time it takes for your
Flutter app to start, you can run the flutter run command
with the trace-startup and profile options.

The trace output is saved as a JSON file called start_up_info.json
under the build directory of your Flutter project.
The output lists the elapsed time from app startup to these trace
events (captured in microseconds):

Time to enter the Flutter engine code.

Time to render the first frame of the app.

Time to initialize the Flutter framework.

Time to complete the Flutter framework initialization.

For example:

Tracing Dart code