Fwd: [OT][X-POST] RAID-6 hw rebuild speed

Sorry for the OT and X-POST but these 2 lists are full of skilled
storage engineer.
For a very,very,very,very long time I used 15k SAS 3.5'' disks. A
RAID-6 hardware (8 disks) took about 20 hours to rebuild.

Now I've replaced a 3.5 disks with a 15k SAS 2.5'' disk. raid is
rebuilding properly, but the ETA is less then 1 hours.

I've moved from a 20 hours rebuild to about 50 minutes rebuild, by
just changing one 3.5' disks with a 2.5'

Is this normal ? I'm thinking something strange is happening