it’s impossible to know what is going on without more details.
how/what is the shield hooked up to
is it using wifi or gigabit ethernet
what does the plex server dashboard show, is direct playing both video and audio, or transcoding?
where are the client and/or server logs ?
you can paste or drag/drop logs zip and screenshots into a reply to help
have you read Plex, 4k, transcoding, and you ???
