diff --git a/mitogen/master.py b/mitogen/master.py
index 2fc1da4f..4dcb70f1 100644
--- a/mitogen/master.py
+++ b/mitogen/master.py
@@ -140,7 +140,7 @@ def get_child_modules(path):
     return [to_text(name) for _, name, _ in it]
 
 
-def get_core_source():
+def _get_core_source():
     """
     Master version of parent.get_core_source().
     """
@@ -150,7 +150,7 @@ def get_core_source():
 
 if mitogen.is_master:
     # TODO: find a less surprising way of installing this.
-    mitogen.parent.get_core_source = get_core_source
+    mitogen.parent._get_core_source = _get_core_source
 
 
 LOAD_CONST = dis.opname.index('LOAD_CONST')
@@ -823,13 +823,14 @@ class ModuleResponder(object):
 
     def _send_load_module(self, stream, fullname):
         if fullname not in stream.sent_modules:
-            LOG.debug('_send_load_module(%r, %r)', stream, fullname)
             tup = self._build_tuple(fullname)
             msg = mitogen.core.Message.pickled(
                 tup,
                 dst_id=stream.remote_id,
                 handle=mitogen.core.LOAD_MODULE,
             )
+            LOG.debug('%s: sending module %s (%.2f KiB)',
+                      stream.name, fullname, len(msg.data) / 1024.0)
             self._router._async_route(msg)
             stream.sent_modules.add(fullname)
             if tup[2] is not None:
diff --git a/mitogen/parent.py b/mitogen/parent.py
index 0f6b4901..661082d6 100644
--- a/mitogen/parent.py
+++ b/mitogen/parent.py
@@ -155,8 +155,8 @@ def get_sys_executable():
     return '/usr/bin/python'
 
 
-_core_source_cache = None
 _core_source_lock = threading.Lock()
+_core_source_partial = None
 
 
 def _get_core_source():
@@ -168,22 +168,24 @@ def _get_core_source():
     return inspect.getsource(mitogen.core)
 
 
-def get_core_source():
+def get_core_source_partial():
     """
     _get_core_source() is expensive, even with @lru_cache in minify.py, threads
     can enter it simultaneously causing severe slowdowns.
     """
-    global _core_source_cache
-    if _core_source_cache is not None:
-        return _core_source_cache
+    global _core_source_partial
 
-    _core_source_lock.acquire()
-    try:
-        if _core_source_cache is None:
-            _core_source_cache = _get_core_source()
-        return _core_source_cache
-    finally:
-        _core_source_lock.release()
+    if _core_source_partial is None:
+        _core_source_lock.acquire()
+        try:
+            if _core_source_partial is None:
+                _core_source_partial = PartialZlib(
+                    _get_core_source().encode('utf-8')
+                )
+        finally:
+            _core_source_lock.release()
+
+    return _core_source_partial
 
 
 def get_default_remote_name():
@@ -572,6 +574,26 @@ def write_all(fd, s, deadline=None):
         poller.close()
 
 
+class PartialZlib(object):
+    def __init__(self, s):
+        self.s = s
+        if sys.version_info > (2, 5):
+            self._compressor = zlib.compressobj(9)
+            self._out = self._compressor.compress(s)
+            self._out += self._compressor.flush(zlib.Z_SYNC_FLUSH)
+        else:
+            self._compressor = None
+
+    def append(self, s):
+        if self._compressor is None:
+            return zlib.compress(self.s + s, 9)
+        else:
+            compressor = self._compressor.copy()
+            out = self._out
+            out += compressor.compress(s)
+            return out + compressor.flush()
+
+
 class IteratingRead(object):
     def __init__(self, fds, deadline=None):
         self.deadline = deadline
@@ -1300,11 +1322,12 @@ class Stream(mitogen.core.Stream):
         }
 
     def get_preamble(self):
-        source = get_core_source()
-        source += '\nExternalContext(%r).main()\n' % (
-            self.get_econtext_config(),
+        suffix = (
+            '\nExternalContext(%r).main()\n' %\
+            (self.get_econtext_config(),)
         )
-        return zlib.compress(source.encode('utf-8'), 9)
+        partial = get_core_source_partial()
+        return partial.append(suffix.encode('utf-8'))
 
     def start_child(self):
         args = self.get_boot_command()