It'll be the transcoding that is the issue. Whether that is because you don't have a direct path, bandwidth issues, or because of codec issues.
Not all devices support all codecs. Any disagreement means that Plex is now going to try transcoding, even if the hardware capability isn't there (and most NASs don't have the grunt to do it).
You have options.
Spend more disk space: You can tell Plex to transcode copies into lower bitrate versions it makes them available via the UI (although this isn't always obvious how, and varies on platform). Note that on a NAS this might take a day to transcode a movie. During which your ability to use the NAS for anything else is greatly reduced.
Spend more effort: Re-rip or re-encode everything into a known supported format. You will need to maintain this. Tools like Tdarr may help here.
More compute power (and some effort):
If your content is in 1080p and you want to view it in 1080p or lower then reasonably-modern Intel CPUs with support for QuickSync are pretty good at handling transcoding for 1080p. Plex Media server has an option to enable hardware-accellerated transcoding if you pay for Plex.
An older Intel NUC or Small-form-factor PC can usually do this job fine. They can also be fairly power efficient.
If you want to start throwing in 4k content, then you are going to need a GPU and the cost and complexity goes up.
Not all devices support all codecs. Any disagreement means that Plex is now going to try transcoding, even if the hardware capability isn't there (and most NASs don't have the grunt to do it).
You have options.
Spend more disk space: You can tell Plex to transcode copies into lower bitrate versions it makes them available via the UI (although this isn't always obvious how, and varies on platform). Note that on a NAS this might take a day to transcode a movie. During which your ability to use the NAS for anything else is greatly reduced.
Spend more effort: Re-rip or re-encode everything into a known supported format. You will need to maintain this. Tools like Tdarr may help here.
More compute power (and some effort):
If your content is in 1080p and you want to view it in 1080p or lower then reasonably-modern Intel CPUs with support for QuickSync are pretty good at handling transcoding for 1080p. Plex Media server has an option to enable hardware-accellerated transcoding if you pay for Plex.
An older Intel NUC or Small-form-factor PC can usually do this job fine. They can also be fairly power efficient.
If you want to start throwing in 4k content, then you are going to need a GPU and the cost and complexity goes up.