The DataGrid select all button loading slow

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WPF

DataGrid

I have a DataGrid with ItemsSource and columns with bound values.
For some reason the "select all button" is loading last, so firstly the column headers are aligned to the left and afterwards the "select all button" appears. How can I fix that?

http://i.stack.imgur.com/EhesT.png[^]

<DataGrid ItemsSource="{Binding ReferenceData}"
                         IsReadOnly="True"
                         >

                   <DataGrid.RowHeaderTemplate>
                       <DataTemplate>
                           <TextBlock Text="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type DataGridRow}},
                               Path=Item.RowHeader}"
                                      HorizontalAlignment="Stretch"
                                      MinWidth="55"/>
                       </DataTemplate>
                   </DataGrid.RowHeaderTemplate>

                   <DataGrid.Columns>
                       <!--m-->
                       <DataGridTextColumn Binding="{Binding Path=m, IsAsync=True, StringFormat='{}{0:0.##}'}" Header="m"/>

                       <!--α-->
                       <DataGridTextColumn Binding="{Binding Path=a, IsAsync=True}" Header="α" />

                       <!--h-->
                       <DataGridTextColumn Binding="{Binding Path=h, IsAsync=True, StringFormat='{}{0:0.#}'}" Header="h" />

                       <!--D1-->
                       <DataGridTextColumn Binding="{Binding Path=D1, IsAsync=True, StringFormat='{}{0:F0}'}">
                           <DataGridTextColumn.Header>
                               <StackPanel Orientation="Horizontal">
                                   <TextBlock>
                                       D
                                       <InlineUIContainer BaselineAlignment="Subscript">
                                           <TextBlock Text="1"/>
                                       </InlineUIContainer>
                                   </TextBlock>
                               </StackPanel>
                           </DataGridTextColumn.Header>
                       </DataGridTextColumn>

                       <!--D2-->
                       <DataGridTextColumn Binding="{Binding Path=D2, IsAsync=True, StringFormat='{}{0:F0}'}">
                           <DataGridTextColumn.Header>
                               <StackPanel Orientation="Horizontal">
                                   <TextBlock>
                                       D
                                       <InlineUIContainer BaselineAlignment="Subscript">
                                           <TextBlock Text="2"/>
                                       </InlineUIContainer>
                                   </TextBlock>
                               </StackPanel>
                           </DataGridTextColumn.Header>
                       </DataGridTextColumn>

                   </DataGrid.Columns>

               </DataGrid>