This article (archive.org) said that there were a few points where Python 3.0 was actually slower than Python 2.6, though I think many of these issues were resolved. That being said, Numpy hasn’t been brought over to Python 3.0 yet and that’s where a lot of the high performance (written in c) number functionality stuff is hiding. Hopefully it will be ready late 2009 or early 2010.
You should not consider performance to be a justification to switch to Python 3; I don’t think you’ll see a consistent speed improvement.
Edit: Versions of Numpy which support Python 3 have since been released.
Edit2: This answer (and other answers to this question) are outdated.