Hi @Jeff_S,
This is an excellent question you’ve posed and, while delayed, I wanted to make sure you saw an official staff response in full detail.
What matters much more than the library size is a) the breakdown of content within the library and b) the associated storage file structure that Roon indexes during use. If your machine has sufficient processing, RAM, and storage, then 1,000,000 tracks in a library is completely possible.
Users who have large libraries with highly customized file directories are currently experiencing issues with performance. Roon expects subdirectories to be loosely structured according to a general music taxonomy, something similar in ratio to Artist - Album - Track. While flexible in its handling, Roon will currently struggle when a directory is taxonomized too far outside that expected structure. To give an example, a single folder containing (possibly hundreds of) live performances of one jazz standard by one artist across two dozen albums isn’t a far-fetched taxonomy. However, under certain conditions, building that directory:subdirectory ratio can bottleneck Roon’s backend processes.
Since users with large Roon local libraries tend to catalog and organize their local libraries, these users have encountered symptoms first.
We’re working to resolve the underlying symptoms to restore functionality in full to these users. Much of this can be fixed to make large libraries and custom taxonomy work better.
However:
Roon is not professional file management software. It is essential that your friend considers how they are organizing their local library and the work that taxonomy will demand of Roon. Keep in mind that TIDAL itself advertises a 90-million track catalog.
With the proper management of storage, Nucleus+ is a wonderful candidate to meet their needs and, with the above notes in mind, can certainly handle large libraries. Your friend can also trial Roon on their current machine, opening access to Community, where they can pose questions to #support directly as well.