diff --git a/mercurial/configitems.py b/mercurial/configitems.py
--- a/mercurial/configitems.py
+++ b/mercurial/configitems.py
@@ -1793,6 +1793,14 @@
 coreconfigitem(
     b'storage', b'revlog.nodemap.mode', default=b'compat', experimental=True
 )
+# experimental as long as format.use-persistent-nodemap is.
+coreconfigitem(
+    b'storage',
+    b'revlog.persistent-nodemap.slow-path',
+    default=b"allow",
+    experimental=True,
+)
+
 coreconfigitem(
     b'storage',
     b'revlog.reuse-external-delta',
diff --git a/mercurial/helptext/config.txt b/mercurial/helptext/config.txt
--- a/mercurial/helptext/config.txt
+++ b/mercurial/helptext/config.txt
@@ -1953,6 +1953,18 @@
     For details on the "persistent-nodemap" feature, see:
     :hg:`help config format.use-persistent-nodemap`.
 
+``revlog.persistent-nodemap.slow-path``
+    Control the behavior of Merucrial when using a repository with "persistent"
+    nodemap with an installation of Mercurial without a fast implementation for
+    the feature:
+
+    ``allow``: Silently use the slower implementation to access the repository.
+
+    Default to "allow"
+
+    For details on the "persistent-nodemap" feature, see:
+    :hg:`help config format.use-persistent-nodemap`.
+
 ``revlog.reuse-external-delta-parent``
     Control the order in which delta parents are considered when adding new
     revisions from an external source.