diff --git a/hgext3rd/undo.py b/hgext3rd/undo.py
--- a/hgext3rd/undo.py
+++ b/hgext3rd/undo.py
@@ -21,6 +21,7 @@
     hg,
     localrepo,
     lock as lockmod,
+    merge,
     obsolete,
     obsutil,
     phases,
@@ -63,38 +64,50 @@
     if 'CHGINTERNALMARK' in os.environ:
         return orig(lui, repo, cmd, fullargs, *args)
 
-    # This wrapper executes whenever a command is run.
-    # Some commands (eg hg sl) don't actually modify anything
-    # ie can't be undone, but the command doesn't know this.
     command = [cmd] + fullargs
 
-    # Check wether undolog is consistent
-    # ie check wether the undo ext was
-    # off before this command
-    if '_undologactive' not in os.environ:
-        changes = safelog(repo, [""])
-        if changes:
-            _recordnewgap(repo)
+    # Only write undo log if we know a command is going to do some writes. This
+    # saves time calculating visible heads if the command is read-only (ex.
+    # status).
+    #
+    # To detect a write command, wrap all possible entries:
+    #  - transaction.__init__
+    #  - merge.update
+    triggered = []
+    def trigger(orig, *args, **kwargs):
+        # trigger a log of the initial state of a repo before a command tries
+        # to modify that state.
+        if not triggered:
+            triggered.append(True)
 
-    # prevent nested calls
+            # Check wether undolog is consistent
+            # ie check wether the undo ext was
+            # off before this command
+            changes = safelog(repo, [""])
+            if changes:
+                _recordnewgap(repo)
+
+        return orig(*args, **kwargs)
+
+    # If this hg process is started by another hg process, do not log from the
+    # inner one, since the inner one is expected to be part of the outer one.
+    # Note: chg server's case is handled above so "hg serve" won't reach here.
     if '_undologactive' not in os.environ:
         os.environ['_undologactive'] = "active"
-        rootlog = True
-    else:
-        rootlog = False
+        extensions.wrapfunction(transaction.transaction, '__init__', trigger)
+        extensions.wrapfunction(merge, 'update', trigger)
 
     try:
         result = orig(lui, repo, cmd, fullargs, *args)
     finally:
-        if repo is not None:
-            # mercurial is bad with caches
-            # eg update to hidden commit will leave false cache
+        # record changes to repo
+        if triggered:
+            if repo is not None:
+                # mercurial is bad with caches
+                # eg update to hidden commit will leave false cache
                 repo.invalidatevolatilesets()
-
-        # record changes to repo
-        if rootlog:
             safelog(repo, command)
-            del os.environ['_undologactive']
+        del os.environ['_undologactive']
 
     return result
 
diff --git a/tests/test-undo.t b/tests/test-undo.t
--- a/tests/test-undo.t
+++ b/tests/test-undo.t
@@ -210,10 +210,32 @@
   $ hg log -G -r 'olddraft(1) and public()' -T compact
 
 Test undolog lock
-  $ hg log --config hooks.duringundologlock="sleep 1" > /dev/null &
-  $ sleep 0.1
-  $ hg st --time
-  time: real [1-9]*\..* (re)
+  $ cat > $TESTTMP/noop.py <<EOF
+  > from __future__ import absolute_import
+  > from mercurial import registrar
+  > cmdtable = {}
+  > command = registrar.command(cmdtable)
+  > @command('noop')
+  > def noop(ui, repo):
+  >     with repo.lock(), repo.transaction('noop'):
+  >         # transaction triggers undolog
+  >         pass
+  > EOF
+  $ cat >> $HGRCPATH <<EOF
+  > [extensions]
+  > noop=$TESTTMP/noop.py
+  > EOF
+  $ hg noop --config hooks.duringundologlock="sleep 1" > /dev/null &
+  $ sleep 0.2
+
+ "hg status" does not trigger undo log writing
+  $ hg status --time
+  time: real 0* (glob)
+
+ remove lock intentionally to test the undolog lock during transaction init
+  $ rm -f .hg/store/lock
+  $ hg noop --time
+  time: real [1-9]\..* (re)
 
 hg undo command tests
   $ hg undo
@@ -431,12 +453,8 @@
 
 File corruption handling
   $ echo 111corruptedrevlog > .hg/undolog/index.i
-#if chg
-(note: chg has issues with the below test)
-#else
-  $ hg st --debug
+  $ hg noop --debug
   caught revlog error. undolog/index.i was probably corrupted
-#endif
   $ hg debugundohistory -l
   0:  -- gap in log --