What is the difference between StatefullWidget and StatelessWidget regards to Performance?

Performance-wise, a StatelessWidget is almost identical to a StatefulWidget with an empty State.

Writing:

class Example extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container();
  }
}

or:

class Example extends StatefulWidget {
  @override
  _ExampleState createState() => _ExampleState();
}

class _ExampleState extends State<Example> {
  @override
  Widget build(BuildContext context) {
    return Container();
  }
}

won't have any visible impact on the performance of your app.

There indeed is a small gain when using StatelessWidget here. But it's ridiculously small:

The difference between them can be summarized as calling an empty function vs not calling it. It's something, but absolutely doesn't matter.

---

The reason being, internally the implementation of StatefulWidget and StatelessWidget is almost the same.

StatelessWidget does have all the extra life-cycles that StatefulWidget possess. It has an "initState"/"dispose". Even a setState! They are just not part of the public API.