Async keyword introduce huge performance decrease

I am trying to understand async/await but I discovered this. The two code below only differ from the keyword async

void main() {
  asyncAwait();
}

void asyncAwait() {
  int i = 0;
  DateTime now = DateTime.now();
  print(" Now ");
  while (i++ < 10000000000);
  DateTime after = DateTime.now();
  print("Done in ${after.difference(now)}");
}

Now Done in 0:00:09.222742

void main() {
  asyncAwait();
}

void asyncAwait() async {
  int i = 0;
  DateTime now = DateTime.now();
  print(" Now ");
  while (i++ < 10000000000);
  DateTime after = DateTime.now();
  print("Done in ${after.difference(now)}");
}

Now Done in 0:00:30.555924

One is at least 3 times faster than the other. I am looking for an explanation at the compiler level but I could not find any documentation about so.

I run it with this command with dart 2.13.0

dart main.dart

If you don't see a huge difference, chance 10000000000 to 100000000000