Was on Exchange 2003SP2 Enterprise on Server 1. Saw backup speeds of over 2GB/min. Server needed to be replaced.
Server 2 enters the scene. Pretty beefy, this time has SAS drives instead of SCSI. Go from an IBM x236 to an IBM x3650. Moved mailboxes through Exchange interface, and everything transitions over fine.
Install Exchange Agent for ArcServe backup, and it installs the Exchange Premium Add-on. Configure backup job, just to backup the information store (not doing brick level). Seeing speeds of 160MB/min. On the Information Store. Both server 1 and 2 were/are connected to the same network segment, same switch as backup server. All 3 are/were connected via Gigabit ports. End result, backup window went from approx 4-5 hours to 40-50 hours. Can't even complete it in a weekend (we're talking over 300GB of email).
ArcServe will backup straight files from the server at approx 1200-1500MB/min. I could reasonably understand 1000-1300MB/min due to the overhead and accessing MAPI (being very generous), but not sure why it's coming up so slowly. The disk activity lights on Server2 don't seem to indicate they're doing a ton of reading and trying to send it across the network to the backup server. Perfmon didn't seem to indicate any high number of MAPI sessions open, or high disk queue. Processor not over utilized, so it doesn't look like it's trying to process anything.