Super Duper and NAS

[Boomer]

Wanted to do a backup of my machine before installing Leopard, but running into the problem that SD is very slow when backing up to my NAS.

Have a Linkstation Pro, and SD shows in its status window a transferspeed of about 3-4MB/s, the activitymonitor shows 7-8MB/s.
Fact is that it takes around 10 hours to backup my 100GB HD.

Copying a 4GB dmg file to the Linkstation gives a transferspeed of 18-20MB/s from the activitymonitor.

Why the big difference in speed between a SD backup and a straight copy?

A backup to a external FW400 drive took about 3 hours.

Any ideas how to speed that up?
The NAS is backed up to a sparse image, the firewire drive was just copied over by SD.