As a sometimes Python core developer allow me to say stuff. The main goal was to remove mistakes; an all-at-once deprecation. The release was actually much less ambitious than the original name "Python 3000" implied. Guido read one of Joel's posts on rewriting software [the one on the Mozilla rewrite] and dictated that 3.0 be done in a year or two. See the list of PEPs [Python Enhancement Proposals] for silly big changes that were shot down.

As for python 3.0 being 10% slower than 2.5 - that means that it is is just 1.9 times faster than Ruby instead of a clean 2x? And the backwards incompatible version took just two years instead of infinity like Perl and two years instead of the UNK for Ruby? Python 2.5 benefited from a corporate sponsored "Need For Speed" sprint where twenty of the core devs went on a paid vacation with a mission to just sprint on speed issues. There hasn't been a corresponding sponsored event for 3.0.

A huge amount of volunteer time has been spent on backwards compatibility issues. When the Python 3.0 issue was first floated the Twisted and Django folks had lots of worries about the transition and supporting two versions (2.x and 3.x). Hundreds of hours were spent on the email lists and PyCon dinners hashing out the details. Thousands of hours were spent adding warnings and best practices docs to 2.6 as well as on the "2to3" translation tool.

Time will tell but there will certainly be lessons learned from the Python 3 release.