Why is Wpf's DrawingContext.DrawText so expensive?

In Wpf (4.0) my listbox (using a VirtualizingStackPanel) contains 500 items. Each item is of a custom Type

class Page : FrameworkElement
...
protected override void OnRender(DrawingContext dc)
{
   // Drawing 1000 single characters to different positions
   //(formattedText is a static member which is only instantiated once and contains the string "A" or "B"...)
   for (int i = 0; i < 1000; i++)
     dc.DrawText(formattedText, new Point(....))


  // Drawing 1000 ellipses: very fast and low ram usage
    for (int i = 0; i < 1000; i++)     
    dc.DrawEllipse(Brushes.Black, null, new Point(....),10,10)


}

Now when moving the scrollbar of the listbox back and forth so that every item's visual is created at least once the ram usage goes up to 500 Mb after a while and then - after a while - goes back to ca 250 Mb but stays on this level. Memory leak ? I thought the advantage of a VirtualizingStackPanel is that visuals which are not needed/visible get disposed...

Anyway, this extreme ram usage only appears when drawing text using "DrawText". Drawing other objects like "DrawEllipse" does not consume so much memory.

Is there a more efficient way to draw many text items than using Drawing.Context's "DrawText" ?

Here is the complete sample (just create a new Wpf Application project and replace the window1 code): (I know there are FlowDocument and FixedDocument but they are no alternative) Xaml:

<Window x:Class="WpfApplication1.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="Window1" Height="900" Width="800">
<Grid Background="Black">
    <ListBox Name="lb" ScrollViewer.CanContentScroll="True"   Background="Black">
        <ListBox.ItemsPanel>
            <ItemsPanelTemplate>
                <VirtualizingStackPanel Orientation="Horizontal" />