Steps to reproduce:

1) Download attached project and open in Unity
2) Make sure Development Build and Autoconnect Profiler are enabled
3) Build and Run 'main' scene on a device
4) Press on button '1'
5) Expand GUI.ProcessEvents >> test.OnGUI() and you will see that Loading.LoadFileHeaders takes ~ 150-170.ms of CPU

Expected result: using Resources.Load() loading time in Loading.LoadFileHeaders should take way much less time (check attached image 'fixed.png')
Actual result: using Resources.Load() loading time in Loading.LoadFileHeaders takes ~ 150-170.ms (check attached image 'actual.png')

Reproduced with:
5.5.0p1 ~ 170.ms
5.5.3p4 ~ 180.ms
5.6.1p1 ~ ~198.ms

Not reproduced with:
2017.1.0b7 ~ 1.6.ms
2017.2.0a1 ~ 1.5.ms

Devices under test:

Xiaomi Mi 4i*, OS:5.0.2, CPU:arm64-v8a, GPU:Adreno (TM) 405, Build:Xiaomi/ferrari/ferrari:5.0.2/LRX22G/V8.1.5.0.LXIMIDI:user/release-keys

Samsung SM G925F (Galaxy S6 Edge)*, OS:7.0, CPU:arm64-v8a, GPU:Mali-T760, Build:samsung/zeroltexx/zerolte:6.0.1/MMB29K/G925FXXU5DQA7:user/release-keys

Xiaomi MI PAD*, OS:4.4.4, CPU:armeabi-v7a, GPU:NVIDIA Tegra, Build:Xiaomi/mocha/mocha:4.4.4/KTU84P/V8.2.2.0.KXFCNDL:user/release-keys