I was told that Roon wouldn’t run on my 251,but I thought I would try before spending money upgrading.
I have the basic TS 251,upgraded memory,2x2TB WD HD and a 32GB SD card in the back.
To my delight it runs Roon and I think it works perfect.
I have Roon Server on the NAS,controller on an MacBook and iPhone and end point is a Cambridge 851N streamer.Stored on the NAS are 953 albums/FLACS all have metadata showing it plays selected tracks/albums almost instantly ,creates listing etc everything I think it should be doing.
My question, when if any time will my NAS struggle to cope ?
How will I know and what are the signs ?
Should I upgrade before this happens or wait and see ?
What would be the next level of NAS to upgrade to ?
I always found that NAS like ts251 or 251+ with 8Go of ram was enough to enjoy Roon experience in terms of browsing etc…
Do not expect room correction but for the rest should be enough to start.
As Roon is recommending i3 or i5 the portfolio of NAS could be quite restricted. Perhaps a small fanless computer could be nice (nuc, zotac, etc…) or diy.
If you mean the DSP settings i have been playing around with those too.and they do work.I like the idea of the NAS on all the time rather than a PC.
My MacBook Pro is i5 but having only 250GB SSD.I also have a iMac networked but don’t like the idea of leaving a PC on all the time
How are Mac minis set up ?
Where are the FLacs stored ?
As long as it’s running fine for you – it’s fine. If your library increases to – say – 50K tracks, you will probably notice some slowdown, especially on complex screens like Discover and Focus. For DSP – you’ll probably be OK using PCM. If you ever want to engage in stuff like DSD upsampling, you’ll hit limits.
Roon’s demand for horsepower may (or may not) change in future releases with new functionality, but as for now – you’re fine. 
No one said it wouldn’t work. What has been said is that the experience could be less satisfying. The larger your music collection grows the larger the potential for issues. If it’s working for you today then enjoy it.