Browsing your music in Roon means accessing its database - and Roon Labs has always said that an SSD is necessary for all but a small library.
Audio analysis (figuring out the graph for local material) is done on import, and this is constrained by the CPU, not by disk access.
In a Nucleus, the Roon database lives on the M.2 SSD, together with the ROCK OS and the Roon software. The optional second internal drive (used for local storage of music files) has to be a 2.5" drive that is less than 9.5mm thick. It can be either HDD or SSD technology. The performance of Roon accessing this drive does not dictate a preference; that is down to whether you want a silent drive (SSD) or more storage available within the physical size constraints (also SSD).