Why does plex transcode when devices can naturally play content?

Hi,
Outline of Issue:
I am currently running plex on a Synology NAS drive (DS214 Play), I realise this is not a powerful device but have found that if I stream from plex to the respective plex app on Phone (sony Z5), Sony TV (Sony-Opera), PS4 the stream keeps buffering, as plex is transcoding on the NAS drive (typically 76% CPU usage). If however I stream using the media players build into the devices mentioned the files play perfectly without transcoding (4>21% CPU usage) I believe via plex dlna. Also if I put the file on a USB stick and plug in TV files play perfectly.
I love the plex interface compared to all the others but if I can not play back without buffering whats the point, hopefully there is a simple way to resolve this issue. I can provided more technical information but would appreciate confirmation something is wrong before I start filling this page with loads of technical info. note, all versions of plex are the latest.
Many Thanks
Alan

There are all sort of reasons it could be transcoding. Subtitles, bitrates that exceed network bandwidth, etc. If you’re using wifi, take that out of the picture and hardwire the TV and PS4 to the PMS and see if the buffering goes away.

maybe PLEX and your media player have different ‘decoder’?
I don’t have experience on android, but I know there are apps such as infuse/vlc on iOS that plays pretty much everything, while PLEX still need help on a lot of files.

also, enabling subtitles sometimes forces video transcoding even when video is natively supported on your device.