RAID5 Array rebuilds terribly slowly

Plugged 3 brand-new 1TB Hitachi drives into MD with onboard MCP78S 8200 SATA ports, booted, and started happily building my new RAID5 array.

It had not completed as of last night, and that would put it at 55% after nearly 52 hours...

$: watch cat /proc/mdstat showed all as it should, during a rebuild, except that finish= bounced between 2000 and 3000 minutes, and speed= reflected between 1000 and 3500 K/sec.

Have since cancelled that rebuild, booted MD to a fully up-to-date Kubuntu amd64 LiveUSB key, and as soon as mdadm had been installed (10 minutes ago or so), it picked up the array, and started a rebuild which currently reflects finish=145.5 min / speed=100000+ K/sec

All this being said, I've a difficult time believing that it might be my hardware...