A good read:
http://docs.python.org/whatsnew/2.6.html
This is because often 2.6 is slower than 2.5. http://mail.python.org/pipermail/python-dev/2008-September/0...
Some things are faster, and some are slower. But for the normal benchmarks mentioned in the release notes (for 2.2, 2.3, 2.4, and 2.5), it's slower overall.
$ python2.6 /opt/local/lib/python2.5/test/pystone.py
Pystone(1.1) time for 50000 passes = 0.867314
This machine benchmarks at 57649.2 pystones/second
$ python2.5 /opt/local/lib/python2.5/test/pystone.py
Pystone(1.1) time for 50000 passes = 0.970622
This machine benchmarks at 51513.4 pystones/second