How to properly improve ItemsControl loading and avoid freezing

I'm wondering what it's the right way to improve my ItemsControl loading. I want to show items that are pretty heavy to load (due to complexe XAML) and I don't want my WPF application to freeze. I want my items to show up once rendered and to be able to interact with the other elements of my window at the same time (even if it's a bit slower). Here is some assumptions to begin with:

1. Let's say that I cannot improve the time of my items to load ;
2. I can't use virtualization because I need to see all my items at once ;
3. The view isn't waiting for a background operation to be completed, all the work is done on the UI thread.

I've been able find a workaround but I'm not happy with it, using a BackgroundWorker and a sleep:

MainWindow.xaml

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
        xmlns:local="clr-namespace:WpfApplication1">
    <DockPanel>
        <Button DockPanel.Dock="Bottom" Content="Click me" Click="OnButtonClick"/>

        <ItemsControl ItemsSource="{Binding Items}">
            <!-- Items begin to be loaded once the ItemsControl is loaded -->
            <i:Interaction.Triggers>
                <i:EventTrigger EventName="Loaded">
                    <i:InvokeCommandAction Command="{Binding LoadCommand}" />
                </i:EventTrigger>
            </i:Interaction.Triggers>
            <ItemsControl.ItemTemplate>
                <DataTemplate>
                    <Border Width="25" Height="25" Background="Red" Margin="5">
                        <local:HeavyItem Content="{Binding}" VerticalAlignment="Center" HorizontalAlignment="Center"/>
                    </Border>
                </DataTemplate>
            </ItemsControl.ItemTemplate>
            <ItemsControl.ItemsPanel>
                <ItemsPanelTemplate>
                    <WrapPanel/>
                </ItemsPanelTemplate>
            </ItemsControl.ItemsPanel>
        </ItemsControl>
    </DockPanel>
</Window>

ViewModel.cs

public class ViewModel
{
    public ICommand LoadCommand { get; protected set; }

    public ObservableCollection<int> Items { get; protected set; }

    public ViewModel()
    {
        Items = new ObservableCollection<int>();
        LoadCommand = new DelegateCommand(AsyncLoad);
        //Load();
    }

    protected void Load()
    {
        for (int i = 0; i < 250; i++)
        {
            Items.Add(i);
        }
    }

    protected void AsyncLoad()
    {
        var bk = new BackgroundWorker();
        bk.DoWork += (s, e) =>
        {
            for (int i = 0; i < 250; i++)
            {
                // Sleep 50ms to let the UI thread breeze
                Thread.Sleep(50);
                Application.Current.Dispatcher.Invoke(DispatcherPriority.Render, (Action)(() =>
                {
                    Items.Add(i);
                }));
            }
        };
        bk.RunWorkerAsync();
    }
}

HeavyItem.cs (fake control to simulate a heavy visual)

public class HeavyItem : ContentControl
{
    protected override Size ArrangeOverride(Size finalSize)
    {
        Thread.Sleep(20);
        return base.ArrangeOverride(finalSize);
    }
}

This question offers a similar approach. I don't like this method because:

1. The ViewModel is doing something that the view should do ;
2. I use an arbitrary timer while the item may take more or less time to render depending on the computer.

I think that the best way to do this is to override ItemsControl and tells the UI thread to update after adding an item, but I didn't manage to do it. Any clue or ideas?