diff --git a/source/performance.txt b/source/performance.txt
index 901114758f328de75a8d78780abc886ef1122ad1_c291cmNlL3BlcmZvcm1hbmNlLnR4dA==..617ca383d1210202892b054f60cf6759522a2b02_c291cmNlL3BlcmZvcm1hbmNlLnR4dA== 100644
--- a/source/performance.txt
+++ b/source/performance.txt
@@ -56,6 +56,11 @@
   might be worthwhile to consider rewriting it as a pure Python version,
   using something like ``ctypes``.
 
+* **Missing RPython modules**: A few modules of the standard library
+  (like ``csv`` and ``cPickle``) are in C in CPython, but in pure Python
+  in PyPy.  Sometimes the JIT is able to do a relatively good job, and
+  sometimes not.  In any case it is still slower in PyPy than in CPython.
+
 We generally consider things that are slower on PyPy than CPython bugs
 of PyPy.  In case you find some issue that is not documented here,
 please report it to our `bug tracker`_ for investigation.