C#: True down cast from child to parent using JSON

downcast.cs

using System;
using System.Collections.Generic;
using System.Linq;
using Newtonsoft.Json;

namespace DownCaster
{
    static class Program
    {
        static void Main()
        {
            var child = new Child
            {
                InParentAndChild = "foo",
                OnlyInChild = "bar",
                Items = new List<SubItem>
                {
                    new SubItem
                    {
                        InItemAndItemChild = "fuz",
                        OnlyInSubItem = "baz"
                    }
                }
            };

            Console.WriteLine("Child: " + JsonConvert.SerializeObject(child));

            var parent = child.DowncastToBase();

            Console.WriteLine("Parent: " + JsonConvert.SerializeObject(parent));

            Console.ReadLine();
        }
    }

    public class Parent
    {
        public string InParentAndChild { get; set; }
        public IList<Item> Items { get; set; }
    }

    public class Child : Parent
    {
        public string OnlyInChild { get; set; }
        public new IList<SubItem> Items { get; set; }
    }

    public class Item
    {
        public string InItemAndItemChild { get; set; }
    }

    public class SubItem : Item
    {
        public string OnlyInSubItem { get; set; }
    }

    public static class DowncastExtensions
    {

        public static Parent DowncastToBase(this Child c)
        {
            var json = JsonConvert.SerializeObject(c);
            var parent = JsonConvert.DeserializeObject<Parent>(json);

            parent.Items = c.Items.Select(i => i.DowncastToBase()).ToList();

            return parent;
        }

        public static Item DowncastToBase(this SubItem si)
        {
            var json = JsonConvert.SerializeObject(si);

            return JsonConvert.DeserializeObject<Item>(json);
        }
    }
}

this is very slow