The Shield is an outstanding player for both local and Internet based streaming services. But even the Pro version is limited in its transcoding abilities. Since Roku devices only support a handful of video/audio codecs and containers, and can't play the captions from most ripped DVDs or Blu Ray discs, video transcoding is necessary for most content. If all your media is in a MKV or MP4 container, the video is MPEG2 or AVC (or HEVC if you have a 4K Roku), and the audio is Dolby Digital or PCM, then no transcoding is ever necessary.
If you want captions, the media is in a different container, or you have an unsupported audio or video codec, transcoding is required and the Shield might not be capable of keeping up. Your Plex server needs to be running on an Intel based NAS or a PC (Intel CPU not required) to have the processing power to perform sufficiently. The PC doesn't have to be a powerhouse. I have Plex on two different Windows based PCs, one with an Intel i5 (maybe mid level CPU from 6-7 years ago) and the other with an AMD quad core CPU that's also many years old. Both can stream without issue to my Roku players both on my network and when I'm traveling using hotel WiFi and TVs.
Dan
Roku Community Streaming Expert
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