ThemedDataGrid locks up UI few 2-3 seconds when loading

I am not sure if this is the correct place for a question about the ThemedDataGrid. I have a DataTemplate that associates a Document ViewModel with a Document View (UserControl). The View is pretty simple right now. Here are the importnat bits.

<UserControl ...>
  <datagrid:ThemedDataGrid ItemsSource="{Binding DataTable}" />
</UserControl>

When I open this Document, by the ViewModel to the DockSite.DocumentItems, the whole UI will lock for 2-3 seconds as its trying to render the display. The DataGrid is bound to a System.Data.DataTable in my ViewModel. When I change it to the standard WPF DataGrid there is no lag when its being opened. Am I doing something wrong? Is this a known issue?

Thanks,

   -eric