1.  Clock-speed 2. Cores   Your disk speed will determine the number of simultaneous tracks you can run. If you're working at 44.1k or 48k, a conventional HD can sustain over 100 solid/contiguous tracks. IOW, It's probably not much of a limitation.  😉   If you're working at 192k, you'll want the extra speed of SSD. As a point of reference: Conventional HD sustains ~200MB/Sec SATA SSD sustains ~540MB/Sec M.2 Ultra SSD sustains ~3500MB/Se