Tweak comments on performance impact based on current implementation

pep-0377.txt

@@ -166,7 +166,7 @@ then handles and suppresses::
 
 With the proposed semantic change in place, the contextlib based examples
 above would then "just work", but the class based version would need
-adjustment to take advantage of the new semantics::
+a small adjustment to take advantage of the new semantics::
 
   class CM(object):
     def __init__(self):
@@ -220,17 +220,31 @@ the same semantic differences as are seen with the ``combined()`` context
 manager in the above example.
 
 
+Performance Impact
+==================
+
+Implementing the new semantics makes it necessary to store the references
+to the ``__enter__`` and ``__exit__`` methods in temporary variables instead
+of on the stack. This results in a slight regression in ``with`` statement
+speed relative to Python 2.6/3.1. However, implementing a custom
+``SETUP_WITH`` opcode would negate any differences between the two
+approaches (as well as dramatically improving speed by eliminating more
+than a dozen unnecessary trips around the eval loop).
+
+
 Reference Implementation
 ========================
 
-In work.
+Patch attached to Issue 5251 [1]. That patch uses only existing opcodes
+(i.e. no ``SETUP_WITH``).
 
 
 Acknowledgements
 ================
 
-James William Pye both raised the issue and suggested the solution
+James William Pye both raised the issue and suggested the basic outline of
-described in this PEP.
+the solution described in this PEP.
+
 
 References
 ==========
@@ -244,6 +258,7 @@ References
 .. [3] Import-style syntax to reduce indentation of nested with statements
    (http://mail.python.org/pipermail/python-ideas/2009-March/003188.html)
 
+
 Copyright
 =========