When you configured the bios / disks, did you do fast initialization? Could have had the raid being built while you wrote to the disk system. Also look into the battery-backed write cache. Lot of times when you order DLs they don't ship with the BBWC which can affect disk performance, I don't see that causing a 2 hour install though.
Check your disks to be sure you don't have a degraded array. You could also do a memtest on the system, though with the install the bulk of the work is disk i/o. I've also seen bad CD drives cause poor install performance, you could try using an external CD or if you have the ILO available, do an ILO remote CD install.
