

My NAS is on an embedded Xeon that at this point is close to a decade old and one of my proxmox boxes is on an Intel 6500t. I’m not really running anything on any really low spec machines anymore, though earlyish in the pandemic I was running boinc with the Open Pandemics project on 4 raspberry pis.
I would double and triple check your not transcoding. Even if you’re watching on the client at whatever the files native resolution is depending on the codec of the file you might still be transcoding. For instance with 1080p anything h265 or AV1 is transcoded into h264 by the server. There’s also a few other situations where Plex with force transcoding or down convert the video whether you want it to or not.
Your NAS shouldn’t be having trouble serving the file to Plex I’d bet it’s transcoding in the background and you just don’t realize it.