Isn’t the core issue the fact that Plexs player is still based on OpenGL and they have no intention on fixing that anytime soon.
The fact that cpu is going that high when this should be something the gpu should be taking care of tells you that right there.
Anything can make almost anything throttle out when not programmed properly.
The fact the Plex player is hitting performance limits is known and so is the reason for it thanks to those pushing Plex for answers for LITERAL years.
This is why infuse works perfect and Plex as a modern media player on appleTV is a joke.