Reproduction steps:
1) Download and open attached project (AVPTest-Unity5.6.2)
2) Build and run "AVPTest-Cylinder and Stereo" scene (inside Assets folder)
3) Using adb logcat pay attention to VrApi messages, which are telling video stats
-- Notice that sometimes FPS drops from 57 - 60 down to 35 - 45, or even lower.

Expected results:
FPS should not drop. Expected fps 57 - 60 (as before regression)

Regression appeared in 5.6.1p1
Reproduced with: 5.6.1p1, 5.6.3f1, 2017.1.0p3, 2017.2.0b3
Unable to reproduce with: 5.6.1f1

Devices under testing:
Reproduced with:
Samsung Galaxy S7 (G930V - Verizon; Qualcomm ), OS:7.0, CPU:arm64-v8a, GPU:Adreno (TM) 530, Build:Verizon/heroqltevzw/heroqltevzw:7.0/NRD90M/G930VVRU4BQA4:user/release-keys

Unable to reproduce with:
Samsung Galaxy S7 (G930F - Exynos), OS:7.0, CPU:arm64-v8a, GPU:Mali-T880, Build:samsung/heroltexx/herolte:7.0/NRD90M/G930FXXU1DQD7:user/release-keys
Galaxy S6 Edge (G925F), OS:7.0, CPU:arm64-v8a, GPU:Mali-T760, Build:samsung/zeroltexx/zerolte:7.0/NRD90M/G925FXXU5EQFC:user/release-keys

Tested with Daydreaam - Google Pixel, OS:7.1.2, CPU:arm64-v8a, GPU:Adreno (TM) 530, Build:google/sailfish/sailfish:7.1.2/NJH47B/4021576:user/release-keys