diff -r d031609b3cb7 -r 33275ab5e837 mercurial/revlog.py
--- a/mercurial/revlog.py	Tue Feb 06 19:08:25 2018 -0800
+++ b/mercurial/revlog.py	Tue Feb 13 11:35:32 2018 -0800
@@ -407,6 +407,9 @@
         for candidaterevs in self._getcandidaterevs(p1, p2, cachedelta):
             nominateddeltas = []
             for candidaterev in candidaterevs:
+                # no delta for rawtext-changing revs (see "candelta" for why)
+                if revlog.flags(candidaterev) & REVIDX_RAWTEXT_CHANGING_FLAGS:
+                    continue
                 candidatedelta = self._builddeltainfo(revinfo, candidaterev, fh)
                 if revlog._isgooddeltainfo(candidatedelta, revinfo.textlen):
                     nominateddeltas.append(candidatedelta)
@@ -2090,7 +2093,14 @@
             deltacomputer = _deltacomputer(self)
 
         revinfo = _revisioninfo(node, p1, p2, btext, textlen, cachedelta, flags)
-        deltainfo = deltacomputer.finddeltainfo(revinfo, fh)
+
+        # no delta for flag processor revision (see "candelta" for why)
+        # not calling candelta since only one revision needs test, also to
+        # avoid overhead fetching flags again.
+        if flags & REVIDX_RAWTEXT_CHANGING_FLAGS:
+            deltainfo = None
+        else:
+            deltainfo = deltacomputer.finddeltainfo(revinfo, fh)
 
         if deltainfo is not None:
             base = deltainfo.base