My knowledge is that an is .ISO on the harddisk is quicker than a real CD or DVD.
The ISO doesn't have such a long access-time in a virtual disc-drive since it's on your harddisk and your mainboard doen't have to handle the communication between the harddisk and the disc-drive via the different busses.
Having an ISO they are on the same medium.
k:
I hope this is understandable...
EDIT: ... and completely correct.
