diff -r ce77436162a5 -r 38f92d12357c mercurial/copies.py
--- a/mercurial/copies.py	Tue Aug 11 15:06:02 2015 -0400
+++ b/mercurial/copies.py	Tue Jan 27 11:26:27 2015 -0800
@@ -191,6 +191,9 @@
     return cm
 
 def _backwardrenames(a, b):
+    if a._repo.ui.configbool('experimental', 'disablecopytrace'):
+        return {}
+
     # Even though we're not taking copies into account, 1:n rename situations
     # can still exist (e.g. hg cp a b; hg mv a c). In those cases we
     # arbitrarily pick one of the renames.
@@ -264,6 +267,12 @@
     if c2.node() is None and c1.node() == repo.dirstate.p1():
         return repo.dirstate.copies(), {}, {}, {}
 
+    # Copy trace disabling is explicitly below the node == p1 logic above
+    # because the logic above is required for a simple copy to be kept across a
+    # rebase.
+    if repo.ui.configbool('experimental', 'disablecopytrace'):
+        return {}, {}, {}, {}
+
     limit = _findlimit(repo, c1.rev(), c2.rev())
     if limit is None:
         # no common ancestor, no copies
@@ -513,7 +522,12 @@
     copies between fromrev and rev.
     '''
     exclude = {}
-    if skiprev is not None:
+    if (skiprev is not None and
+        not repo.ui.configbool('experimental', 'disablecopytrace')):
+        # disablecopytrace skips this line, but not the entire function because
+        # the line below is O(size of the repo) during a rebase, while the rest
+        # of the function is much faster (and is required for carrying copy
+        # metadata across the rebase anyway).
         exclude = pathcopies(repo[fromrev], repo[skiprev])
     for dst, src in pathcopies(repo[fromrev], repo[rev]).iteritems():
         # copies.pathcopies returns backward renames, so dst might not