Just to add to the discussion too... My Exynos S8 has also become useless. 6k videos don't play anymore and 5k videos stutter after a few mins.
This happened after what I think was an android update.
Some people have mentioned that it's actually the Wifi card throttling itself after the phone decides it's overheated. If you do a speedtest after playing 5k video for a few mins you'll noticed the considerably dropped speeds.
Here's a thread on reddit:
https://www.reddit.com/r/oculusnsfw/comments/bv0chu/s8_cant_play_videos_smoothly_anymore/
I've seen others, but that's the one where they discuss the issue.
This sucks because it used to work great.