diff --git a/mercurial/util.py b/mercurial/util.py
--- a/mercurial/util.py
+++ b/mercurial/util.py
@@ -38,13 +38,16 @@ def _fastsha1(s):
 
 import subprocess
 closefds = os.name == 'posix'
-def popen2(cmd, bufsize=-1):
-    p = subprocess.Popen(cmd, shell=True, bufsize=bufsize,
+def popen2(cmd):
+    # Setting bufsize to -1 lets the system decide the buffer size.
+    # The default for bufsize is 0, meaning unbuffered. This leads to
+    # poor performance on Mac OS X: http://bugs.python.org/issue4194
+    p = subprocess.Popen(cmd, shell=True, bufsize=-1,
                          close_fds=closefds,
                          stdin=subprocess.PIPE, stdout=subprocess.PIPE)
     return p.stdin, p.stdout
-def popen3(cmd, bufsize=-1):
-    p = subprocess.Popen(cmd, shell=True, bufsize=bufsize,
+def popen3(cmd):
+    p = subprocess.Popen(cmd, shell=True, bufsize=-1,
                          close_fds=closefds,
                          stdin=subprocess.PIPE, stdout=subprocess.PIPE,
                          stderr=subprocess.PIPE)