diff --git a/mercurial/context.py b/mercurial/context.py
--- a/mercurial/context.py
+++ b/mercurial/context.py
@@ -766,10 +766,17 @@ class basefilectx(object):
         # fetch the linkrev
         fr = filelog.rev(fnode)
         lkr = filelog.linkrev(fr)
+        # hack to reuse ancestor computation when searching for renames
+        memberanc = getattr(self, '_ancestrycontext', None)
+        iteranc = None
+        if memberanc is None:
+            memberanc = iteranc = cl.ancestors([srcrev], lkr,
+                                               inclusive=inclusive)
         # check if this linkrev is an ancestor of srcrev
-        anc = cl.ancestors([srcrev], lkr, inclusive=inclusive)
-        if lkr not in anc:
-            for a in anc:
+        if lkr not in memberanc:
+            if iteranc is None:
+                iteranc = cl.ancestors([srcrev], lkr, inclusive=inclusive)
+            for a in iteranc:
                 ac = cl.read(a) # get changeset data (we avoid object creation)
                 if path in ac[3]: # checking the 'files' field.
                     # The file has been touched, check if the content is
@@ -826,6 +833,8 @@ class basefilectx(object):
                 rev = self._adjustlinkrev(path, l, fnode, self.rev())
                 fctx = filectx(self._repo, path, fileid=fnode, filelog=l,
                                changeid=rev)
+                fctx._ancestrycontext = getattr(self, '_ancestrycontext', None)
+
             else:
                 fctx = filectx(self._repo, path, fileid=fnode, filelog=l)
             ret.append(fctx)
diff --git a/mercurial/copies.py b/mercurial/copies.py
--- a/mercurial/copies.py
+++ b/mercurial/copies.py
@@ -170,8 +170,11 @@ def _forwardcopies(a, b):
     missing = set(b.manifest().iterkeys())
     missing.difference_update(a.manifest().iterkeys())
 
+    ancestrycontext = a._repo.changelog.ancestors([b.rev()], inclusive=True)
     for f in missing:
-        ofctx = _tracefile(b[f], am, limit)
+        fctx = b[f]
+        fctx._ancestrycontext = ancestrycontext
+        ofctx = _tracefile(fctx, am, limit)
         if ofctx:
             cm[f] = ofctx.path()