ListBox filter performance

Hi,

I am populating a (surface) listbox with about 1200 items of the following DataTemplate:

<DataTemplate x:Key="FileListDataTemplate"><StackPanel Orientation="Horizontal"><MediaElement x:Name="FileThumb" Source="{Binding Path=ShowSelectedMedia}" MinHeight="75" MaxWidth="192" MaxHeight="90"></MediaElement><Label MaxWidth="200" Content="{Binding Path=Filename}" /></StackPanel></DataTemplate>

It is a list of filenames in a folder on a websever and when the users selects one it displays the photo in the listitem that has its filename in the label of the listitem.

There is also a textbox on top of the the listbox which should filter the contents of the listbox and that where the problem arises.

Getting the ICollectionView of the listbox and applying the filter the following way:

filterText = SearchFileTextbox.Text;
ICollectionView cv = CollectionViewSource.GetDefaultView(Filenames);
cv.Filter = o =>
  {
    FTPFileListModel f = o as FTPFileListModel;
    return f.Filename.Contains(filterText);
  };

It results in a UI that hangs for 4-5 seconds when entering the first letters in the textbox. How could I avoid this? A BackgroundWorker? Could someone provide an example?

I have tried to put all this in a BGWorker:

ICollectionView cv = CollectionViewSource.GetDefaultView(Filenames);
cv.Filter = o =>
{
   FTPFileListModel f = o as FTPFileListModel;
   return f.Filename.Contains(filterText);
};

But this doen't update the view, as if I was not writting anything in the textbox although the code gets executed because the breakpoint is hit.

Thanks for any help

Greg