--- a/mercurial/dispatch.py	Wed Feb 15 13:17:39 2017 -0800
+++ b/mercurial/dispatch.py	Wed Feb 15 13:17:45 2017 -0800
@@ -137,6 +137,9 @@
     finally:
         duration = util.timer() - starttime
         req.ui.flush()
+        if req.ui.logblockedtimes:
+            req.ui._blockedtimes['command_duration'] = duration * 1000
+            req.ui.log('uiblocked', 'ui blocked ms', **req.ui._blockedtimes)
         req.ui.log("commandfinish", "%s exited %s after %0.2f seconds\n",
                    msg, ret or 0, duration)
     return ret

--- a/mercurial/ui.py	Wed Feb 15 13:17:39 2017 -0800
+++ b/mercurial/ui.py	Wed Feb 15 13:17:45 2017 -0800
@@ -7,6 +7,7 @@
 
 from __future__ import absolute_import
 
+import collections
 import contextlib
 import errno
 import getpass
@@ -138,6 +139,8 @@
         self.callhooks = True
         # Insecure server connections requested.
         self.insecureconnections = False
+        # Blocked time
+        self.logblockedtimes = False
 
         if src:
             self.fout = src.fout
@@ -155,6 +158,7 @@
             self.fixconfig()
 
             self.httppasswordmgrdb = src.httppasswordmgrdb
+            self._blockedtimes = src._blockedtimes
         else:
             self.fout = util.stdout
             self.ferr = util.stderr
@@ -164,6 +168,7 @@
             self.environ = encoding.environ
 
             self.httppasswordmgrdb = httppasswordmgrdbproxy()
+            self._blockedtimes = collections.defaultdict(int)
 
         allowed = self.configlist('experimental', 'exportableenviron')
         if '*' in allowed:
@@ -192,6 +197,15 @@
             self._progbar.resetstate()  # reset last-print time of progress bar
         self.httppasswordmgrdb = httppasswordmgrdbproxy()
 
+    @contextlib.contextmanager
+    def timeblockedsection(self, key):
+        starttime = util.timer()
+        try:
+            yield
+        finally:
+            self._blockedtimes[key + '_blocked'] += \
+                (util.timer() - starttime) * 1000
+
     def formatter(self, topic, opts):
         return formatter.formatter(self, topic, opts)
 
@@ -295,6 +309,7 @@
             self._reportuntrusted = self.debugflag or self.configbool("ui",
                 "report_untrusted", True)
             self.tracebackflag = self.configbool('ui', 'traceback', False)
+            self.logblockedtimes = self.configbool('ui', 'logblockedtimes')
 
         if section in (None, 'trusted'):
             # update trust information

--- a/tests/test-logtoprocess.t	Wed Feb 15 13:17:39 2017 -0800
+++ b/tests/test-logtoprocess.t	Wed Feb 15 13:17:45 2017 -0800
@@ -10,6 +10,7 @@
   > def foo(ui, repo):
   >     ui.log('foo', 'a message: %(bar)s\n', bar='spam')
   > EOF
+  $ cp $HGRCPATH $HGRCPATH.bak
   $ cat >> $HGRCPATH << EOF
   > [extensions]
   > logtoprocess=
@@ -52,3 +53,18 @@
   logtoprocess commandfinish output:
   logtoprocess foo output:
   spam
+
+Confirm that logging blocked time catches stdio properly:
+  $ cp $HGRCPATH.bak $HGRCPATH
+  $ cat >> $HGRCPATH << EOF
+  > [extensions]
+  > logtoprocess=
+  > pager=
+  > [logtoprocess]
+  > uiblocked=echo "\$EVENT command \$OPT_COMMAND_DURATION ms"
+  > [ui]
+  > logblockedtimes=True
+  > EOF
+
+  $ hg log
+  uiblocked command [0-9]+.[0-9]* ms (re)