I see the same issue. I had to turn off ZoomEnabled to get it to show up faster. Basically, with ZoomEnabled, you hold down the item the menu is linked to and it takes a good 2 seconds for it to simply appear. There is no animation going on, it just comes up instantly with everything else behind it zoomed back. It closes with an animation though. It's the opening animation that is totally broken. I guess that explains the 2 second delay. It's the animation delay except we're not seeing it.
Setting isZoomEnabled to false makes it work right again just that you lose the nifty animation and zooming.
<toolkit:ContextMenuService.ContextMenu>
<toolkit:ContextMenu IsZoomEnabled="False" >
<toolkit:MenuItem Header="{Binding Converter={StaticResource LocalizedBinder}, ConverterParameter='Edit'}" IsEnabled="{Binding EditCommentGridVisibility}" Click="EditComment_Click" Tag="{Binding commentId}" />
<toolkit:MenuItem Header="{Binding Converter={StaticResource LocalizedBinder}, ConverterParameter='Remove'}" IsEnabled="{Binding RemoveCommentGridVisibility}" Click="DeleteComment_Click" Tag="{Binding commentId}" />
</toolkit:ContextMenu>
</toolkit:ContextMenuService.ContextMenu>