Time to load Cubase is dependent upon the OS drive transfer rate. If you leave all you VST plugins on the OS drive as well, the scanning of those will also be dependent upon that drive.
Cubase itself does not appear to load up complete files at startup, except perhaps for the small image files so that it can display the waveforms. The audio files themselves will probably not be loaded at all, but just the blocks immediately required once playing. Seek and transfer times of the project drive will play a big part in responsiveness when dealing with lots of tracks.
MIDI is included in the project file, which is small enough to be fully loaded.
We have been all SSD for the last few years, but when I got tired of keeping only current projects on the projects drive, and SSDs were now cheaper, I went for a Samsung 250GB 840 EVO to store them all on a dedicated drive, along the VST(i) dlls, rather than a common data drive, which is now a smaller SATA II drive holding less timing-dependant stuff like documents, etc.