Shader effects kill performance of Silverlight popup control.

 Today I found out why all pop ups in our project works so slowly (As an example you can check datepicker on this page http://househotel74.ru/#/Booking).

The source of problem is silverlight shader effects. If you have control with applied shader effect behind popup it will work very slowly.

You can check a sample project to make your own experiments.

May be anybody knows the nature of this problem?