So, we have had this: The 1000% Speedup, or, the stdlib sucks. It demonstrates a rather bad bug that is probably costing the universe a load of cycles even as we speak. It’s fixed now, which is great.
So what parts of the standard library have you noticed to be evil?
I would expect all the responsible people to match up an answer with a bug report (if suitable) and a patch (if superman).
(since this is a different module, placing it in a different answer)
cgitb has some weird threading issues. See this bug report.