diff --git a/mercurial/exchange.py b/mercurial/exchange.py
--- a/mercurial/exchange.py
+++ b/mercurial/exchange.py
@@ -522,8 +522,21 @@
 
 def _checksubrepostate(pushop):
     """Ensure all outgoing referenced subrepo revisions are present locally"""
+
+    repo = pushop.repo
+
+    # If the repository does not use subrepos, skip the expensive
+    # manifest checks.
+    try:
+        if not len(repo.file(b'.hgsub')) or not len(repo.file(b'.hgsubstate')):
+            return
+    except RuntimeError:
+        # Alternative filelog implementations might not implement this,
+        # so just fallback to the generic implementation.
+        pass
+
     for n in pushop.outgoing.missing:
-        ctx = pushop.repo[n]
+        ctx = repo[n]
 
         if b'.hgsub' in ctx.manifest() and b'.hgsubstate' in ctx.files():
             for subpath in sorted(ctx.substate):