Keeping in mind that in 2007 we use EWS, you've got to think of the migration as running via HTML. So network efficiency and I/O are the crucial factors.
Russ produced this list of limiting factors (in priority order)
- Speed of the SAN (Mostly it's I/O bound)
- Speed of the NETWORK (Next, it's related to how fast HTTPS requests are
- Speed of the SERVERS (CAS, then MBX) (Third, how fast the Web Services Requests are PROCESSED)
- Speed of the 32-bit (Fourth, how fast those requests are GENERATED)
In Exchange 2000/2003, where we used CDO, the horsepower of the individual back end Exchange server played a more crucial role.