How to reproduce:
1. Open the attached project or create a new one
2. Make sure you set Development build and enable Profiler
3. Build and run test scene (it is the same empty scene Unity creates by default) on Android device
4. Check the profiler
Result: On this specific device skybox rendering takes rather long ~20ms in empty scene

Reproducible: 2017.2.0b1, 2017.1.0b10, 5.6.2f1, 5.5.4f1

DUT(reproducible):
91, Google Galaxy Nexus [maguro], 4.3, PowerVR, TI OMAP

DUT(not reproducible):
69, Meizu MX5, 5.0.1, PowerVR, MediaTek (Skybox rendering takes ~0.52ms)

BY Design: The skybox is procedural = the shader is very heavy, so it's normal that it's slow on older devices.