That's actually one of the more interesting parts about Angular. I'm not sure of the details but I believe the Angular team have been counting on Object.observe for increasing the performance of the Angular code [0] (something that Google / the Chrome team have influence over).
The Angular code you write now won't need to be updated and will get a great speed boost later. Other libraries, like Ember, get you to add all that metadata to your code to give you a performance boost now (which to me feels like something you'll regret in a year's time).
Actually, I'm sure it'll break a few things. Angular doesn't have a good concept of update granularity, which can be problematic when you get to the multi-directive interaction phase...
Google staffs the project with several talented engineers. Misko Hevery is an amazing benevolent dictator. He earns my trust and respect. A confluence of genius and pragmatism.