A more philosophical question, but… I’m curious if is it possible to optimize rendering system by NOT rendering 3D scene behind 100% opaque UI elements? With UI like e.g. minimap, skill bars, hp/mana bars, etc., it could improve FPS on many games.
Even if not in UE, is it implemented like that anywhere? Or maybe is it just impossible? 
I’m also interested in this. Currently it looks like everything in UMG/UI elements appear to be rendered as translucent, even 100% opaque elements, so they are not effectively masking the rendered scene, and in fact hurt performance, especially on mobile.
Can someone at Epic confirm this?
It would be great to be able to set UI elements to be 100% opaque (where alpha is completely ignored) so that no elements behind the UI are rendered.
Additionally, if textures are used instead of materials, there is no way to set UI elements to use masked rendering, which Epic has recommended as being more performant than translucent rendering.
Lastly, I have seen suggestions that translucent rendering is actually more efficient on mobile/tile-based architectures. See alpha blend vs. alpha test from Nikolas Smedberg’s PDF - can Nikolas comment on this? Is this still relevant in UE4/on current hardware? How does Metal handle translucency? Alpha Blend vs Alpha Test
I’m still curious if this is even possible in any 3D engine.
That’s interesting, I didn’t know that UMG elements are drawn as translucent. Have you checked it with some performance comparisons with UMG UI on/off?
Regarding this quite old topic of mine:
Weird optimization idea: Instead of having HUD elements as UMG widgets, use 3D meshes with Opaque material as UI elements and align them from 3D to screen space in Blueprints (simplest way, just attach them to the camera).
This way they should occlude a good amount of screen with light green shader complexity 
And I guess that scene behind them should be culled. Not sure if that makes sense, maybe it’s worth trying.