diff -r 47d2664b1da4 Lib/http/client.py
--- a/Lib/http/client.py	Mon Dec 31 21:44:06 2012 -0600
+++ b/Lib/http/client.py	Tue Jan 01 20:34:10 2013 +1100
@@ -719,6 +719,12 @@
     default_port = HTTP_PORT
     auto_open = 1
     debuglevel = 0
+    # TCP Maximum Segment Size (MSS) is a TCP stack parameter.  There
+    # is no simple and efficient platform independent mechanism for
+    # determining the MSS, so instead a reasonable estimate is chosen.
+    # The getsockopt() interface using the TCP_MAXSEG parameter may
+    # be a suitable approach on some operating systems.
+    mss = 2048
 
     def __init__(self, host, port=None, strict=_strict_sentinel,
                  timeout=socket._GLOBAL_DEFAULT_TIMEOUT, source_address=None):
@@ -886,8 +892,11 @@
         del self._buffer[:]
         # If msg and message_body are sent in a single send() call,
         # it will avoid performance problems caused by the interaction
-        # between delayed ack and the Nagle algorithm.
-        if isinstance(message_body, bytes):
+        # between delayed ack and the Nagle algorithm. However,
+        # there is no performance gain if the message is larger
+        # than MSS (and there is a memory penalty for the message
+        # copy).
+        if isinstance(message_body, bytes) and len(message_body) < self.mss:
             msg += message_body
             message_body = None
         self.send(msg)
diff -r 47d2664b1da4 Misc/ACKS
--- a/Misc/ACKS	Mon Dec 31 21:44:06 2012 -0600
+++ b/Misc/ACKS	Tue Jan 01 20:34:10 2013 +1100
@@ -705,6 +705,7 @@
 Tshepang Lekhonkhobe
 Marc-André Lemburg
 John Lenton
+Benno Leslie
 Christopher Tur Lesniewski-Laas
 Alain Leufroy
 Mark Levinson