Considering the lifespan of iSCSI when compared to standard SCSI or Fibre Channel, it's not uncommon for people to be unfamiliar with the proper configuration and speed expectations of the new protocol.
iSCSI is an IP based standard that uses the 1Gb/s and 10Gb/s Ethernet standards to transfer data. Many newer SANs have opted to use iSCSI instead of the costlier Fibre Channel.
There are two ways to operate an iSCSI network:
From my personal experience, I have compiled my average real-world backup speeds to an LTO3 tape drive that I've been able to achieve when backing up over iSCSI on a 1Gb/s network:
Backing up Exchange Mail stores (disjoint / low-traffic network): Average of 30-50 MB/sec / 1.7-2.9 GB/min / 102-174 GB/hr
Backing up Exchange Mail stores (multipurpose network): Average of 20-40 MB/sec / 1.2-2.3 GB/min / 72-138 GB/hr
Backing up File Servers (disjoint / low-traffic network): Average of 25-60 MB/sec / 1.4-3.5 GB/min / 84-210 GB/hr
Backing up File Servers (multipurpose network): Average of 15-40 MB/sec / 0.8-2.3 GB/min / 48-138 GB/hr
Obviously, depending on the size of your Exchange mail stores and the sizes and types of files on your File Servers can alter the backup speeds that you can achieve.