ListBox slow even with non-active Groupstyle

I have noticed that my ListBox is very slow even if I have non-active GroupStyle. The difference with 1000 items are seconds!

<ListBox...><ItemsControl.GroupStyle ><GroupStyle></GroupStyle></ItemsControl.GroupStyle></ListBox>

Is that a bug or am I missing something here?

I have attached a sample Application that demonstrates this. http://sdrv.ms/13urdMg