What are some costly performance operations that I need
to be careful with?
    
      
Opacity, Clip.antiAliasWithSaveLayer,
 or anything that triggers saveLayer

      ImageFilter
      Also see Performance best practices

How do I tell which widgets in my Flutter app are rebuilt
in each frame?
    
      Set debugProfileBuildsEnabled true in
widgets/debug.dart.
      Alternatively, change the performRebuild function in
widgets/framework.dart to ignore
debugProfileBuildsEnabled and always call
Timeline.startSync(...)/finish.
      If you use IntelliJ, a GUI view of this data is available.
Select show widget rebuild information,
and you’ll visually see which widgets rebuild
visually in your IDE.

How do I query the target frames per second (of the display)?
    
      Get the display refresh rate