Software RAID 6 initial sync very slow

Hi all,

I am debating between hardware or software RAID for a high-performance 
Linux storage server. One area of concern in my initial mdadm tests 
concerns sync speed. My setup is as follows:

Linux Fedora 7 x64
12 x 1TB Hitachi 1TB SATAII drives connected via LSI SAS3801E PCIe HBA
Dual 2.5GHz quad-core Xeon
8GB RAM

My first test was to create a RAID 6 md device, 128K chunk, with all 12 
drives. The sync is currently underway at a very slow 30MB/sec, with 
mdadm estimating this as a 9hr process. With an Adaptec 5805 RAID card 
tested, I performed the RAID 6 initial sync with the same 12 drives in 
just under 5 hours. Meanwhile, our total CPU usage during mdadm syncing 
is at 5%, so there's plenty of CPU not being used.

Why is the sync so slow? This concerns me not only because initial sync 
time is important, but because this may be indicative of slow sync 
times in the event of a drive failure.

Thanks for your input,
Thomas