Problem with constant Unaccounted fps reducer.

I'm struggling with a weird low fps problem. I have only 30 objects on scene, which are quads (or planes, or cubes doesn't matter, I've tried to change meshes but there was no much difference) to make ground. Anyway, only ~2k triangles (with quads). I've made an Android build (tested on Lenovo Vibe X2 and some other devices), and get ~30 fps on Lenovo, and ~17 fps on other devices with almost empty scene. Textures, materials, shaders, lighting... all doesn't make any difference. The problem only in those 30 quads, because when I add some other models on scene (min 1k triangles each model, > 20 models), fps decreases only by ~1-3 points. But when deactivate the ground, I get a constant 60 fps.

Profiler shows, that the problem is in Unaccounted area, sometimes up to 90%, so I can't find the source of it. Unity version 5.4.1 The only similar problem I found here: http://answers.unity3d.com/questions/1235376/profiler-constant-unaccounted-entry-takes-50-13ms.html

But I have my problem on all devices (so far), and the problem disappears if I deactivate ground. And that's only for mobile build, on PC it works flawless.

Any ideas what might it be and/or how to avoid it?