Thanks for the reply. I went hunting and found a wiki page on mobile performance. In render settings I disabled Separate Translucency (even though I believe I had no translucency in this to begin with). I made custom materials for the 3 materials. They are marked fully rough with lightmap directionality also turned off. I also deleted the box brush that was used as a backdrop and replaced it with a 2 tri plane. None of these materials have textures, only color. None of the materials change over time.
I did find out how to show the FPS. It’s 23 on the device (a visible improvement from what it was before, but I don’t have a figure for what it was before).
Still this is: 9 cubes * 108 triangles + 2 (974 total) for the backdrop and absolutely no textures and as far as I can tell no postprocessing (although I really don’t know if there’s a global disable option) and minimal lighting and shader features afaik as well.
This device is certainly not top of the light, but it’s no slouch: it runs Epic Citadel at an average of 55 FPS (which was UDK 3).
I converted this simple thing to run via Unity and it gets 60FPS (vsynced). I understand there can be some differences between engines or my lack of familiarity with UE4 causing me to kill performance on mobile, but if so I’m missing it!