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Yep, different devices have different capabilities to play back files natively. If the device can't play back the file natively (doesn't have the processing power or doesn't have the codec) then plex will either repackage it (doesn't take much processing power) or transcode it (takes a lot of processing power).
This gives you a bit of an explanation
https://techuplife.com/best-plex-client-2020/If you're looking at the cheap end of the scale then a Raspberry Pi 4 will probably do the job as a client but will take some set up.
I'd agree with Velocio that I'd go for a separate cheap NAS and then something else for playback/Plex server.
I've got an Nvidia Shield Pro (has to be the pro) too and it's good. Really good interface and snappy compared to a lot of similar devices. I've never really used the Plex server properly so can't comment on that though (you will need an external SSD or flash drive if you want to run a plex server which is about as far as I got).
The other option is an Intel NUC. A second hand one for about the same price as a Shield will have plenty of grunt for transcoding (if you want to watch on mobile for instance) and work as a client. It's swings and roundabouts which you prefer, a PC offers more flexibility but you may need to get a keyboard and mouse out every so often.
We might not be contradicting each other.
For pure Plex on an LAN there's no need to transcode.
But if the Roku clients ask for a transcode then Plex is going to transcode.