Dart await extremely slow with HttpRequest

Question

Say I have an endpoint (I'll call it SERVICE) that returns a simple JSON string. In Dart, the following code is consistently very fast; it runs in less than a second:

main() => HttpRequest.getString(SERVICE).then(print);

However, this code is consistently about 10 times slower:

main() async => print(await HttpRequest.getString(SERVICE));

Why is this so much slower? Am I doing something wrong here?

If it matters, I'm testing this in Dartium using the simple "bare bones web application" example in WebStorm, and I get the same behavior in Chrome.