I mentioned this issue in a private conversation with Carl and he suggested that I send the details to the mailinglist. The following blog post is a nice summary of a significant performance problem in IronPython's early implementation with good links to the key discussions: http://ironpython-urls.blogspot.com/2009/05/python-jython-and-ironpython.htm... Robert Smallshire implemented OWL Basic in IronPython and experienced some massive slowdowns relative to CPython and Jython. IronPython developer Dino Diehland investigated the problem and wrote an detailed blog post about the problem and the fix implemented in IronPython. The problem was due to the user program's use of a class variable as a counter, which caused "unfortunate" behavior in the IronPython implementation -- mutating the variable invalidated the cache. PyPy shows a good speedup compared with CPython already, but it is an interesting corner case. The smaller testcase created by Robert might be a good benchmark to add to PyPy Speed Center (http://www.smallshire.org.uk/sufficientlysmall/2009/05/22/ironpython-2-0-and...). - David