It's slow. Real slow. The original demo really stressed Ember's pathological cases, so it was a great thing to keep in mind as we developed Glimmer.
I saw the original at a Meetup a while back and it was crash-the-browser slow. This is far improved.
I dont know if this is exactly the same app, but it looks like it: http://youtu.be/z5e7kWSHWTg?t=5m07s (ember is on the left, then angular, then react)
They showed that side by side. Imagine the page reloading every 2 seconds instead of at the rate it is now. The improvement is incredible to see.