Hard drive speed really has no influence in anything besides saving and load times or if your pc has so little ram that it resorts to using the slow swap file.
Granted, during the "live" game play, the hard drive is not as much of a factor. However, drive speed is a critically severe bottleneck with respect to load/saving/building lots, and any transitions between them. Those are the performances areas that most desperately need improvement, ergo they *are* a factor.