I always add :
-webkit-backface-visibility: hidden;
-webkit-perspective: 1000;
When working with 3d transform. Even “fake” 3D transforms. Experience tells me that these two lines always improve performances, especially on iPad but also on Chrome.
I did test on your exemple and, as far as I can tell, it works.
As for the “why” part of your question… I don’t know. 3D transform are a young standard, so implementation is choppy. That’s why it’s a prefixed property : for beta testing. Someone could fill a bug report or a question and have the team investigate.
Edit per August 19th 2013:
There’s regular activity on this answer, and I just lost an hour finding that IE10 also need this.
So don’t forget :
backface-visibility: hidden;
perspective: 1000;