I occasionally do benchmarks; here's the code I use to do them.
Python JavaScript
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css_
json_
.gitignore
README.rst
bench.py
css.js
css2.js
css_big.js
json.js

README.rst

To use:

./bench.py json_
./bench.py css_

I'm terribly concerned w/ making this usable, but feel free to make an issue if you have a suggestion.