In a WPF ListBox with more than 1000 Image Items the Zoom Images become slow

<DataTemplate DataType="{x:Type modeldata:ImageInfo}">
        <Image
            Margin="6"
            Source="{Binding Thumbnail}"
            Width="{Binding ZoomBarWidth.Width, Source={StaticResource zoombarmanager}}"
            Height="{Binding ZoomBarWidth.Width, Source={StaticResource zoombarmanager}}"/>
  </DataTemplate>

<Grid DataContext="{StaticResource imageinfolder}">
    <ScrollViewer
        VerticalScrollBarVisibility="Auto"
        HorizontalScrollBarVisibility="Disabled">
        <ListBox Name="PhotosListBox"
            IsSynchronizedWithCurrentItem="True"
            Style="{StaticResource PhotoListBoxStyle}"
            Margin="5"
            SelectionMode="Extended"
            ItemsSource="{Binding}"
           />
    </ScrollViewer>

<Style~~ TargetType="{x:Type ListBox}" x:Key="PhotoListBoxStyle">
        <Setter Property="Foreground" Value="White" />
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type ListBox}" >
                    <WrapPanel
                        Margin="5"
                        IsItemsHost="True"
                        Orientation="Horizontal"
                        VerticalAlignment="Top"
                        HorizontalAlignment="Stretch" />
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style~~>

<ListBox.ItemsPanel>
   <ItemsPanelTemplate>
      <WrapPanel Orientation="Horizontal"/>
   </ItemsPanelTemplate>
</ListBox.ItemsPanel>

<Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Disabled" />

<ListBox.ItemsPanel>
   <ItemsPanelTemplate>
      <StackPanel>
        <StackPanel.LayoutTransform>
           <ScaleTransform
               ScaleX="{Binding Path=Value, ElementName=ZoomSlider}"
               ScaleY="{Binding Path=Value, ElementName=ZoomSlider}" />
        </StackPanel.LayoutTransform>
      </StackPanel>
   </ItemsPanelTemplate>
</ListBox.ItemsPanel>

class PathToThumbnailConverter : IValueConverter {
    public int DecodeWidth {
        get;
        set;
    }

    public PathToThumbnailConverter() {
        DecodeWidth = 200;
    }

    public object Convert( object value, Type targetType, object parameter, System.Globalization.CultureInfo culture ) {
        var path = value as string;

        if ( !string.IsNullOrEmpty( path ) ) {

            FileInfo info = new FileInfo( path );

            if ( info.Exists && info.Length > 0 ) {
                BitmapImage bi = new BitmapImage();

                bi.BeginInit();
                bi.DecodePixelWidth = DecodeWidth;
                bi.CacheOption = BitmapCacheOption.OnLoad;
                bi.UriSource = new Uri( info.FullName );
                bi.EndInit();

                return bi;
            }
        }

        return null;
    }

    public object ConvertBack( object value, Type targetType, object parameter, System.Globalization.CultureInfo culture ) {
        throw new NotImplementedException();
    }

}