PEP 572: Add a short section on performance

pep-0572.rst

@@ -84,6 +84,23 @@ avoided where regular assignment can be used, just as ``lambda`` should be
 avoided when ``def`` is an option.
 
 
+Performance costs
+=================
+
+The cost of SLNBs must be kept to a minimum, particularly when they are not
+used; the normal case MUST NOT be measurably penalized.  SLNBs are expected
+to be uncommon, and using many of them in a single function should definitely
+be discouraged.  Thus the current implementation uses a linked list of SLNB
+cells, with the absence of such a list being the normal case. This list is
+used for code compilation only; once a function's bytecode has been baked in,
+execution of that bytecode has no performance cost compared to regular
+assignment.
+
+Other Python implementations may choose to do things differently, but a zero
+run-time cost is strongly recommended, as is a minimal compile-time cost in
+the case where no SLNBs are used.
+
+
 Open questions
 ==============