Flat design lets you make smaller 9 slices I guess. Smaller textures take up less ram but even that's a stretch.

I wasn't talking about icons alone in my second statement. :/

The design trend require soft shadows to be rendered on many elements in layers which I exaggeratedly pointed out. I apologise.

Shadows lead to overdraw I suppose, but I don't believe most UI frameworks draw front to back anyway. They end up accelerating it by making large, multi-element bitmaps of the final render. It ends up being a very small cost.