Seems like those my stutter issues are related to shadows. Getting normally 8ms-10ms hit from dynamic light shadows, but time to time those values get suddently many times bigger (also possible the basepass does). Had dynamic shadows disabled in editor for around hours while working in editor (not playing once). Then when I toggled those back the FPS went to 1 per ~3seconds. With shadows off the editor stays smooth and stable (over 50fps all the time), but with shadows there is clear performance leak going (fps is between 40 and 0,2, with same location and camera angle). Feels like the shadow system is leaking down very quick.
i7 5820K, 32Gt DDR2400, GTX780, Win10 64bit.
Edit: Checked it again. Suddently dynamic light shadows jumped to over 200ms and basepass to 40ms, without a reason.