DocumentWindow.Activate takes long time

<Window
    x:Class="ActivateWindow.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:docking="http://schemas.actiprosoftware.com/winfx/xaml/docking"
    Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition />
            <ColumnDefinition />
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition />
            <RowDefinition Height="auto" />
        </Grid.RowDefinitions>
        <docking:DockSite Name="docksite">
            <docking:Workspace>
                <docking:TabbedMdiHost>
                    <docking:TabbedMdiContainer />
                </docking:TabbedMdiHost>
            </docking:Workspace>
        </docking:DockSite>
        <Grid x:Name="DockingGrid" Grid.Column="1" />
        <Button Grid.Row="1" Content="Create new DocumentWindow with content" Margin="2" Click="ShowDocumentWindow" />
        <Button Grid.Row="1" Grid.Column="1" Content="Create content directly" Margin="2" Click="SetGridChild" />
    </Grid>
</Window>

using System.Diagnostics;
using System.Windows;
using System.Windows.Controls;
using ActiproSoftware.Windows.Controls.Docking;

namespace ActivateWindow
{
    public partial class MainWindow : Window
    {
        public MainWindow() { InitializeComponent(); }

        private void ShowDocumentWindow(object sender, RoutedEventArgs e)
        {
            MyUserControl myUserControl = new MyUserControl();

            // Create a DocumentWindow and set its content
            DocumentWindow documentWindow = new DocumentWindow(docksite, "window", "window", null, null);
            documentWindow.Content = myUserControl;

            // Time the DocumentWindow.Activate function
            Stopwatch stopwatch = Stopwatch.StartNew();
            documentWindow.Activate();
            MessageBox.Show(stopwatch.Elapsed.ToString(), "Layout Time: DocumentWindow.Activate");
        }

        private void SetGridChild(object sender, RoutedEventArgs e)
        {
            MyUserControl myUserControl = new MyUserControl();

            // Time the WPF engine layout with built in controls
            Stopwatch stopwatch = Stopwatch.StartNew();
            DockingGrid.Children.Clear();
            DockingGrid.Children.Add(myUserControl);
            MessageBox.Show(stopwatch.Elapsed.ToString(), "Layout Time: WPF standard controls");
        }
    }

    public class MyUserControl : UserControl
    {
        public MyUserControl()
        {
            StackPanel sp = new StackPanel();
            for (int i = 0; i < 2000; i++)
                sp.Children.Add(new TextBlock { Text = "TextBlock " + i });
            this.Content = new ScrollViewer { Content = sp };
        }
    }
}