In my case id used the wrong driver for the JMicron Esata controller embedded on ASUS motherboard.
I where using a driver with WriteCache disabled, changed to "JMB36X_WinDrv_R1.17.49_WHQL_eSATA" and now Backupexec reports 1750Mb/min with software compression.
First i tested drive-speed with "HD Tune 2.55" and it reported 85Mb/sec in average, seems ok.
Then i tested by copying "1,68 GB (1 811 751 635 byte) 1183 files in 30 folders" and it took a staggering 4min 20s /about 400Mb/min // 6,6Mb/s.
That is not good!
With the new driver i copied the same files in 52s /about 1938Mb/min // 32,3Mb/s .
My advice, test speed on Esata by copying a large amount of files.
Kind regards //Dennis