I think I have finally identified the likely cause of this problem, and I have also found a workaround, which I have been testing with positive results during the last couple of days. Mike’s input was essential and our discussion put me on the right track.
I have evidence showing that the cause of the problem is the large number of credits assigned by Roon to each of the albums in the Mozart 225 set: 1215, and how Roon handles these credits. This had been already mentioned in this thread. The presence of these many credits attached to each album appear to heavily impact Roon performance. I could imagine that perhaps it is Roon trying to establish links for the 1215 credits across the 200 albums of the Mozart set whenever you open one of these.
In any case, manually deleting the credits in the album edit window does not solve the problem. This is likely due to the fact that Roon continues to keep track of those credits after deletion (they appear as “removed” in the album screen).
On the other hand, how the albums are physically grouped in folders or organised as separate albums or in collections does not directly impact performance. Nevertheless there can be an indirect effect when albums are put in different sets, as Mike rightly suggested. The reason would be that, based on different sets, Roon can apply different credits to the albums. More on this later.
I had hyphotesized that reducing the number of credits in the Mozart 225 set albums to a more reasonable figure could eliminate the performance issues. But how could this be done, as manually deleting the credits in Roon does not have the desired effect?
The answer was to “identify” each album again in Roon (Album editor / Album options / Identify album), and change the source that Roon uses to obtain its metadata. Luckily, the Mozart 225 set being a compilation, many albums preexisted and could be used in this way for identification. Moreover, DG/Decca had published some time ago the same content split in smaller collections, for downolad. Separate sets existed for Complete Quartets, Complete Symphonies, Early Operas, Serenades & Divertimenti, and the like, abvailable to be used as source for identification in Roon.
See the image below. A CD of the Mozart 225 set containing Mozart’s string quartets is currently getting its metadata from “Mozart 225: The New Complete Edition” (See the message “This is the metadata that is currently being used for This album”). But when “identifying” it again in Roon, I can switch the source, for example, to “Mozart: the String quartets” by the Hagen, or “Mozart 225: Complete Quartets”, which surely will have fewer credits than the Mozart 225.
And indeed, by selecting “Mozart: 225 Complete Quartets”, the number of credits for this album are reduced from the initial 1215 to a much more manageable 66.
Voilà. All the performance issues are gone for each of the albums in the 225 set that I have modified as described above. Until now, I have identified again with this process about 40 albums. All of them load, play, or open for editing without delay.
I have kept all the CDs in the Mozart 225 set as individual albums or small multidisc albums (an opera, for example), just like before, as this greatly facilitates search and use. Only in one case I used a new set (the complete Symphonies). No difference, performance is good all across.
Identifying the albums again has modified the title I had given to them, and probably also some metadata. But I am happy with the result, obviously.
Perhaps having a huge album with all the 200 CDs together in it and only one set of 1215 metadata, as Mike suggested, could also improve Roon performance. But I did not feel like trying it, as it did not seem the kind of organisation I wanted to use. Luckily, I found another way.
I intend to continue down this path to identify again the full collection. As the number of albums still identified according to the big Mozart set goes down, I will recheck performance, to see, for example, what happens when only 50, 20 or 10 albums are left still with the 1215 metadata. I imagine that as the number of albums with the full 1215 metadata diminish, performance when handling them should improve.
It has been an interesting excercise. As Mike said, if Roon developers are looking now into dealing with sets, this one is probably the most challenging they can find to test their solutions.
There are perhaps some lessons to be learned from this. But this can be left for another post.
Thanks to both of you