We recently encountered a performance issue in stdlib for pypy. It turned out that someone commited a performance "fix" that uses += for strings instead of "".join() that was there before.
Now this hurts pypy (we can mitigate it to some degree though) and possible Jython and IronPython too.
How people feel about generally not having += on long strings in stdlib (since the refcount = 1 thing is a hack)?
What about other performance improvements in stdlib that are problematic for pypy or others?
Personally I would like cleaner code in stdlib vs speeding up CPython. Typically that also helps pypy so I'm not unbiased.