Merge pull request realpython#57 from alex/patch-5

Note the speed difference.

Author: Kenneth Reitz

docs/starting/which-python.rst

@@ -66,7 +66,8 @@ PyPy aims for maximum compatibility with the reference CPython implementation
 while improving performance.
 
 If you are looking to squeeze more performance out of your Python code, it's
-worth giving PyPy a try.
+worth giving PyPy a try. On a suite of benchmarks, it's current `over 5 times
+faster than CPython <http://speed.pypy.org/>`_.
 
 Currently PyPy supports Python 2.7.