# UNDF: UNDF-2026-000000039
diff --git a/Lib/pkgutil.py b/Lib/pkgutil.py
--- a/Lib/pkgutil.py
+++ b/Lib/pkgutil.py
@@ -301,6 +301,7 @@ def extend_path(path, name):

     path = path[:] # Start with a copy of the existing path
+    path_set = set(path)  # O(1) membership — avoids O(n²) in the loop below

     parent_package, _, final_name = name.rpartition('.')
     if parent_package:
@@ -320,9 +321,10 @@ def extend_path(path, name):
             for portion in portions:
                 # XXX This may still add duplicate entries to path on
                 # case-insensitive filesystems
-                if portion not in path:
+                if portion not in path_set:
                     path.append(portion)
+                    path_set.add(portion)

         # XXX Is this the right thing for subpackages like zope.app?
         # It looks for a file named "zope.app.pkg"