If too much transcoded is needed to be done and the server is not powerful enough, then the stream pauses (resulting in buffering) until the server is ready to send the next transcoded/processed 'chunk' for streaming towards the plex client. If the plex client doesn't support the content's resolution or codecs for direct streaming, it sends the plex media server a request for transcoding (meaning 'something which can understand' so it can stream it), which results in a heavily-dependent cpu process that causes slow starting times and might cause buffering in some cases. If the plex client fully supports those, the content is directly streamed from the server, resulting in faster loading times and no buffering/pauses due to processing during the stream.

However, the plex client is the one responsible for requesting content by sending the plex server the necessary information regarding whether the resolution and video/audio codecs are supported or not. By installing the plex app at your seedbox, you are creating a new plex server at your plex account in which the remote media will reside and ready to be streamed.

Plex media server roku transcode