Reproduction steps:
1. Open the attached project;
2. Check if "Multithreaded Rendering" is enabled in Player Settings;
3. Build the scene "video" on a device;

Actual result: the video is very laggy.
Expected result: the video should be shown without any disturbances.

Note: if "Multithreaded Rendering" is disabled, the video is shown correctly. Also, the bug does not appear in Editor and standalone.

Reproduced with: 5.6.3f1, 2017.1.0p3, 2017.2.0b6, 2017.3.0a3

Note#2: the video is stopped at all in 2017.2 and 2017.3 versions.

Devices under test:

Google Pixel XL*, OS:8.0.0, CPU:arm64-v8a, GPU:Adreno (TM) 530, Build:google/marlin/marlin:O/OPP2.170420.019/4021594:user/release-keys

Google Nexus 5X*, OS:7.1.2, CPU:arm64-v8a, GPU:Adreno (TM) 418, Build:google/bullhead/bullhead:7.1.2/N2G47W/3938523:user/release-keys

Verizon SM G930V*, OS:7.0, CPU:arm64-v8a, GPU:Adreno (TM) 530, Build:Verizon/heroqltevzw/heroqltevzw:7.0/NRD90M/G930VVRU4BQA2:user/release-keys

Amazon KFMEWI (Fire HD 8 (2015))*, OS:5.1.1, CPU:armeabi-v7a, GPU:PowerVR Rogue G6200, Build:Amazon/full_thebes/thebes:5.1.1/LVY48F/32.5.6.3_user_563231720:user/release-keys