Fredbert thank you very much for your extensive and considered response. I've been away for some months and am home now and contemplating what to do with my NAS and Plex which was completely unavailable for the months I was away (I've never been able to get Plex remote access to work in five years despite extensive help on Plex forums - it does seem that many many users cannot get it to work) and I cannot access the Synology from outside the home (it would be good if I could find a way to do that). Also nothing recorded for two months as the NAS unmounted from the iMac server as soon as I left home. So no Plex again.
The one piece of good news was that I moved the iMac server closer to the NAS and this has improved performance. For example I could previously only play Plex media at mobile phone resolution (with pre-conversion) on my 4K TV which was highly pixelated and unintelligible (I stopped using the setup for a long time because of this as well), but now I get a sparkling 576 picture (I'm told this is still not very good).
I'm thinking of:
1. ditching the NAS and just watching Netflix!
2. replacing my DS1515+, DX513 and new iMac which seem to be hopelessly under-powered for running Plex even without any transcoding, with a
DS1520+ and DX517 and run the Plex Server on the NAS instead of the Mac (to get around the unmount problem). It is scary investing further in Synology when for most of the life of the 1515+ it has not been able to do the job; or
3. Fully investigate existing config as per your pointers.
I am hopeful that the 1520+ might be more up to the job particularly if I avoid transcoding. (I previously once had the server on the 1515+ but it was too weak, so I moved it to the iMac).
I understand that the DX513 is incompatible with the 1520+ which is disappointing as I haven't had the 513 for long? If so I would need the DX517.
Also by having the server on the NAS I would avoid the Mac unmounting problem.
I would pursue your extensive approach to investigating the issues with the existing config, but I don't have the expertise and time is tight. Though ideally this third option is what I should do!
I am wondering if there is some network problem. I used Synology Assistant to load the web interface and it did so quickly today, but when I log into DS Manager I find it takes about three minutes to access - would that be normal? Also when I looked at Package Centre (all packages) it took a long time to load them.
Very interested in your thoughts on my three options.
With thanks, A