Documentation for simplejson mentions that it should be faster. Though it contains reference to Python 2.5, 2.6 so It is unclear whether the claim is outdated:
It is the externally maintained version of the json library contained in Python 2.6, but maintains compatibility with Python 2.5 and (currently) has significant performance advantages, even without using the optional C extension for speedups. </quote> http://simplejson.readthedocs.org/en/latest/index.html
I see there are performance patches to stdlib's json since 2.6: http://hg.python.org/cpython/search/?rev=_json.c&revcoun...
An order of magnitude difference might be an issue with your python installation.
On my machine version from 2.6 (1.9) is 15 times slower than from 2.7 (2.0.9). Though simplejson (2.2.1) is still faster on both 40 and 2 times correspondingly for .loads() timeline.js from the example as measured by:
% timeit json.loads(timeline_text)
it seems that C speedups available on both versions:
python -v -c'import json' 2>&1 | grep _json.so
python -c'import json.decoder as d; print d.scanstring'
Is your python version old? Otherwise it is worth to open issues at
http://bugs.python.org to merge speed improvements and at
https://github.com/simplejson/simplejson/issues to mention in the docs and on
http://pypi.python.org/pypi/simplejson more clearly that Python 2.7+ stdlib's version is also slower.