DS1821+ Plex Server question

I recently bought a DS1821+ NAS so that I could combine my storage of videos’ and Plex Server. I noticed that I still get the message “the server is not fast enough to play this” when playing certain videos. I’m assuming it is because the DS1821+ can not do hardware transcoding? Does anyone know if this is the case and does anyone know of any tweaks that might help other than hosting the plex server on a separate machine? I appreciate any feedback/comments.