DataTemplate for ViewModel slow to display - wpf

Question

I have a simple MainWindowView
<Window ...>
<Window.Resources>
<DataTemplate DataType="{x:Type local:ClassListForTeacherViewModel}">
<local:ClassListForTeacher />
</DataTemplate>
</Window.Resources>
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="109*"/>
<ColumnDefinition Width="408*"/>
</Grid.ColumnDefinitions>
<local:NavigationControl Grid.Column="0"
VerticalAlignment="Stretch"
HorizontalAlignment="Stretch"
Width="Auto"/>
<ContentControl Content="{Binding MainView}"
Grid.Column="1"
VerticalAlignment="Stretch"
HorizontalAlignment="Stretch"
Width="Auto"/>
</Grid>
<Window>
and a MainWindowViewModel
public class MainWindowViewModel : ViewModelBase
{
public MainWindowViewModel(string name)
: this(name, null)
{
}
public MainWindowViewModel(string name, ViewModelBase mainView)
{
Name = name;
MainView = mainView;
NavigateToTeacherClassesCommand = new DelegateCommand(o => NavigateToTeacherClasses());
}
//
// Properties
//
public string Name { get; private set; }
private ViewModelBase _MainView;
public ViewModelBase MainView
{
get
{
return _MainView;
}
private set
{
_MainView = value;
RaisePropertyChangedEvent("MainView");
}
}
public ICommand NavigateToTeacherClassesCommand { get; private set; }
//
// Functions
//
private void NavigateToTeacherClasses()
{
MainView = new ClassListForTeacherViewModel();
}
}
The ClassListForTeacher view only contains a ListView control with no logic (yet). When I click the Label in the NavigationControl in the MainWindowView, the ClassListForTeacher is set on the MainView property of MainWindowView and is displayed via the DataTemplate. This is barebones program; no loading data yet. My problem is the display takes too long, maybe about a full 3 seconds. It is my first time trying out MVVM, I see this is a common technique (using DataTemplate), is this normal?
I also noticed that the ViewModel for ClassListForTeacher is called twice but I have no other code calling it. One from the NavigateToTeacherClasses function, the other I don't know. When WPF actually resolves the DataTemplate (which is a UserControl), is the constructor called again? If so, How do I prevent this?
EDIT:
XAML for NavigationControl
<UserControl...
<StackPanel>
<Expander Header="Grading" IsExpanded="True">
<StackPanel Background="#FFE5E5E5">
<Label Content="My Classes">
<Label.InputBindings>
<MouseBinding Command="{Binding NavigateToTeacherClassesCommand}" MouseAction="LeftClick" />
</Label.InputBindings>
</Label>
</StackPanel>
</Expander>
</StackPanel>
</UserControl>
XAML for ClassListForTeacher:
<UserControl...
<Grid x:Name="LayoutRoot">
<ListView>
<ListView.View>
<GridView>
<GridViewColumn/>
</GridView>
</ListView.View>
</ListView>
</Grid>
</UserControl>