Your video card should support NVENC and NVDEC for decoding H.264 streaming video.
Does it change with resolution? What kind of bandwidth do you have? Is it pixelation, or macroblocking?
https://www.lifewire.com/macroblocking-and-pixelation-1847333
Edit: Also, check the video encode/decode utilization of the GPU in Task Manger. FYI, I'm running the April 2018 edition of Windows 10 (version 1803). So if that option isn't available in Task Manager, you might have to upgrade the OS to get it.