# UNDF: UNDF-2026-000000058
diff --git a/src/EFCore/Metadata/IReadOnlyProperty.cs b/src/EFCore/Metadata/IReadOnlyProperty.cs
index xxxxxxx..xxxxxxx 100644
--- a/src/EFCore/Metadata/IReadOnlyProperty.cs
+++ b/src/EFCore/Metadata/IReadOnlyProperty.cs
@@ -229,19 +229,27 @@ namespace Microsoft.EntityFrameworkCore.Metadata;
     IReadOnlyList<T> GetPrincipals<T>()
         where T : IReadOnlyProperty
     {
         var principals = new List<T> { (T)this };
-        AddPrincipals((T)this, principals);
+        var visited = new HashSet<T>(ReferenceEqualityComparer.Instance) { (T)this };
+        AddPrincipals((T)this, principals, visited);
         return principals;
     }

-    private static void AddPrincipals<T>(T property, List<T> visited)
+    // efcore-0002 fix: pass a HashSet<T> for O(1) duplicate detection.
+    // Previously List<T>: visited.Contains() is O(N) per call, invoked inside
+    // a recursive traversal over foreignKey.Properties × FK chains, giving
+    // O(P²) cost where P = principal chain length.  HashSet reduces to O(P).
+    private static void AddPrincipals<T>(T property, List<T> principals, HashSet<T> visited)
         where T : IReadOnlyProperty
     {
         foreach (var foreignKey in property.GetContainingForeignKeys())
         {
             for (var propertyIndex = 0; propertyIndex < foreignKey.Properties.Count; propertyIndex++)
             {
                 if (ReferenceEquals(property, foreignKey.Properties[propertyIndex]))
                 {
                     var principal = (T)foreignKey.PrincipalKey.Properties[propertyIndex];
-                    if (!visited.Contains(principal))
+                    if (visited.Add(principal))
                     {
-                        visited.Add(principal);
-
-                        AddPrincipals(principal, visited);
+                        principals.Add(principal);
+                        AddPrincipals(principal, principals, visited);
                     }
                 }
             }
         }
     }