I have been using plex on nvidia shield for years and my biggest movie file is about 89gb 4k hdr and it plays beautifully with very little delay starting and stopping. My Server is a hackintosh.
My father wanted the same set up so he tried through his Samsung TV and 2020 imac but it buffered on most things through Ethernet. Then he bought a newer nvida shield tv and when playing through shieldTV from a 2020 imac it played most 1080p but buffered on all 4k files and large 1080p files.
I decided to make the nvidia shield the server cutting out the imac and ethernet connection by plugging a fast USB3 drive directly into the ShieldTV. BUT! it still buffers on the big files even with a direct connection from HDD to ShieldTV.
Any ideas please ? 
Jay
Server Version#:latest
Player Version#:latest