diff --git a/hg b/hg
--- a/hg
+++ b/hg
@@ -28,7 +28,10 @@
 
 # enable importing on demand to reduce startup time
 try:
-    if os.environ.get('HGDEMANDIMPORT') != 'disable':
+    default = ''
+    if 'CHGINTERNALMARK' in os.environ:
+        default = 'disable'
+    if os.environ.get('HGDEMANDIMPORT', default) != 'disable':
         if sys.version_info[0] < 3 or sys.version_info >= (3, 6):
             import hgdemandimport; hgdemandimport.enable()
 except ImportError: