4.6 2D GUI Mask Causing Serious Lag

Hey I'm having problems with masks in the new GUI System in 4.6. They are causing HUGE lag (fps goes down to 8 on a 4 year old iPhone, thats ridiculous!). I think the problem might be that you can't place the sprites-default material to the images inside the mask (adding the material breaks the mask).

Does anyone have a fix or a workaround? This problem has been bugging me for a long time.