You are running into the limitations of the Plex Web and SmartTV clients and running Plex on a Synology NAS.
See this post:
The Plex app on Samsung & LG Smart TVs is also limited when it comes to subtitle support.
Enabling subtitles can result in transcoded video and buffering.
See this post regarding the Plex for LG app. AFAIK, the Samsung app has the same behavior.
Regarding the bandwidth graphs, what you are seeing is normal. Plex does not stream video at a steady rate. It sends the stream to the client as fast as possible until the client buffer is full. Once the buffer empties to a certain level, the client asks for more data and Plex refills the buffer.
Clients have different sized buffers, so some cannot queue up as much data. That is why you see one big spike for Plex for Windows and many smaller spikes for Plex Web.
