title: ObservableCollection clears very slow
body: <p>I have <code>ObservableCollection&lt;MyClass&gt;</code> instance with 20 elements in it and set it as <code>ItemsSource</code> for a control. When I try to call <code>Clear()</code> for this collection, it becomes clean, but after 4-5 seconds of freezing app. If I <em>don`t use it like value for dependency property</em> it clears fast. It also doesn't take any time when I <em>simply run (Ctrl+F5 in Visual Studio)</em> the application instead of debugging.</p>
  
  <p>Never faced with such slowness while working with <code>ObservableCollection</code> and don't understand how to fix this(even <em>while profiling</em> delay is not shown). I develop control with ItemsSource property, so I can't unassign property value - clear collection - set property value again. My custom control's handlers work fast. And app freezes after Clear() call and before my custom <code>INotifyCollectionChanged.CollectionChanged</code> event handler called.</p>