Short version: I am seeing good speeds with latest software.
I had a lot of performance problems with the beta versions of Retrospect 8. I'm just getting set up with the release version. Initial backup speed was awful, but I discovered the external hard drive was showing up as USB 1 which rather crippled performance! Switching it to firewire and running full backups...
Retrospect Server running on iMac with 1 GB RAM (Console v8.0.608.1)
Local backup from internal hard drive to external firewire hard drive
Completed: 540819 files, 119.1 GB
Performance: 860.8 MB/minute (687.7 copy, 1150.2 compare)
Duration: 04:50:18 (00:07:11 idle/loading/preparing)
Backup of client v6.3.019 running on PPC Mac Mini over 100 Mb ethernet
Completed: 340507 files, 30.2 GB
Performance: 210.8 MB/minute (137.0 copy, 456.8 compare)
Duration: 05:05:09 (00:12:05 idle/loading/preparing)