diff -r 5b08e8b7ab00 -r e6067bec18da mercurial/hg.py
--- a/mercurial/hg.py	Wed Oct 10 01:30:45 2012 +0200
+++ b/mercurial/hg.py	Wed Oct 03 13:19:53 2012 -0700
@@ -336,6 +336,16 @@
 
             destlock = copystore(ui, srcrepo, destpath)
 
+            # Recomputing branch cache might be slow on big repos,
+            # so just copy it
+            dstcachedir = os.path.join(destpath, 'cache')
+            srcbranchcache = srcrepo.sjoin('cache/branchheads')
+            dstbranchcache = os.path.join(dstcachedir, 'branchheads')
+            if os.path.exists(srcbranchcache):
+                if not os.path.exists(dstcachedir):
+                    os.mkdir(dstcachedir)
+                util.copyfile(srcbranchcache, dstbranchcache)
+
             # we need to re-init the repo after manually copying the data
             # into it
             destpeer = peer(ui, peeropts, dest)