Ribbon PopupButton leads to slow window resizing and docked-window switching

I am using ActiPro Docking and Ribbon.  I have noticed that after clicking on the Ribbon PopupButton, there is a significant delay when the window is resized, or when switching to a different docking window.  Clicking on a Ribbon Button does not lead to such a delay--only the PopupButton.  I believe the delay is tied to the fact that my window contains a DataGrid with thousands of rows that are virtualized.  However, this large DataGrid does not experience any such delay if the PopupButton has not been clicked.

To reproduce the problem, I started with a fresh WPF Application in VS 2010.  Here is the code for MainWindow.xaml

<ribbon:RibbonWindow
    x:Class="RibbonPopupProblem.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
	xmlns:shared="http://schemas.actiprosoftware.com/winfx/xaml/shared" 
	xmlns:ribbon="http://schemas.actiprosoftware.com/winfx/xaml/ribbon" 
    xmlns:docking="http://schemas.actiprosoftware.com/winfx/xaml/docking"
    Title="MainWindow"
    Height="350" Width="525" >

    <DockPanel LastChildFill="True" >
        <DockPanel DockPanel.Dock="Top" >
            <ribbon:Ribbon 
                x:Name="ribbon"
                UseScenicLayout="False" >
                <ribbon:Ribbon.Tabs >
                    <ribbon:Tab Label="TestTab" >
                        <ribbon:Group Label="TestGroup" >
                            <ribbon:Button Label="Simple Button 1" Margin="7"/>
                            <ribbon:Button Label="Simple Button 1" Margin="7"/>
                            <ribbon:PopupButton Label="Popup Button 1" Margin="7">
                                <StackPanel>
                                    <Label>Dummy 1</Label>
                                    <Label>Dummy 2</Label>
                                    <Label>Dummy 3</Label>
                                </StackPanel>
                            </ribbon:PopupButton>
                        </ribbon:Group>
                    </ribbon:Tab>
                </ribbon:Ribbon.Tabs>
            </ribbon:Ribbon>
        </DockPanel>

        <DockPanel LastChildFill="True" DockPanel.Dock="Bottom" >
            <docking:DockSite DockPanel.Dock="Right" x:Name="MainDockSite" 
                        ItemContainerRetentionMode="Wrapped"
                        CanToolWindowsAutoHide="False"
                        CanDocumentWindowsRaft="False"
                        CanToolWindowsRaft="False">
                <docking:Workspace x:Name="MainWorkSpace">
                    <docking:TabbedMdiHost>

                        <docking:TabbedMdiContainer>
                            <docking:DocumentWindow Title="Test Window 1">
                                <Grid>
                                    <DataGrid 
                                        x:Name="TestDataGrid1" 
                                        AutoGenerateColumns="True"

                                        EnableRowVirtualization="True"
                                        VirtualizingStackPanel.IsVirtualizing="True"
                                        VirtualizingStackPanel.VirtualizationMode="Recycling" />
                                </Grid>
                            </docking:DocumentWindow>
                            <docking:DocumentWindow Title="Test Window 2">
                                <Grid>
                                    <DataGrid 
                                        x:Name="TestDataGrid2" 
                                        AutoGenerateColumns="True"

                                        EnableRowVirtualization="True"
                                        VirtualizingStackPanel.IsVirtualizing="True"
                                        VirtualizingStackPanel.VirtualizationMode="Recycling" />
                                </Grid>
                            </docking:DocumentWindow>
                        </docking:TabbedMdiContainer>

                    </docking:TabbedMdiHost>
                </docking:Workspace>
            </docking:DockSite>
        </DockPanel>

    </DockPanel>

</ribbon:RibbonWindow>

 And here is the code-behind

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;

using ActiproSoftware.Windows.Controls.Ribbon;

namespace RibbonPopupProblem
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : RibbonWindow
    {
        public List<DataItem> _dataItems1 = new List<DataItem>();
        public List<DataItem> _dataItems2 = new List<DataItem>();
        public MainWindow()
        {
            InitializeComponent();

            for (int i = 0; i < 30000; i++)
            {
                _dataItems1.Add(new DataItem { x = i * 2 });
                _dataItems2.Add(new DataItem { x = -i * 6 });
            }

            TestDataGrid1.ItemsSource = _dataItems1;
            TestDataGrid2.ItemsSource = _dataItems2;
        }
    }

    public class DataItem
    {
        public Double x { get; set; }
        public Double y { get { return x / 5; } }
    }
}

Test 1: Start the application.  Do not click on any ribbon buttons.  Maximize the window.  On my computer, this seems to take less than half a second.  Close the application.

Test 2: Start the application. Click on Simple button 1. Maximize the window. On my computer, this seems to take less than half a second. Close the application.

Test 3: Start the application. Click on the PopupButton. Maximize the window. On my computer, this seems to take more than 3 seconds. Close the application.

Test 4: Start the application. Do not click on any ribbon buttons. Switch to Test Window 2. On my computer, this seems instant Close the application.

Test 5: Start the application. Click on Simple button 1. Switch to Test Window 2. On my computer, this seems instant. Close the application.

Test 6: Start the application. Click on the PopupButton. Switch to Test Window 2. On my computer, it seems to take more than 1 second. Close the application.

Of course, this is a stripped-down project to demonstrate the problem.  The delays are a bit more significant in my real project.