Thanks for the advice. I tried removing the drive, reseating it, replaced with an old drive, and it was still very slow. Finally i decided to try replace the sata cable, and success! I guess it was just the cable that was faulty.
It isn't normal. To determine if the 2nd new drive is defective, try to:
1. Remove the 2nd new drive an replace it by an old drive to see if the rebuild speed is normal (if it is, then let it complete);
2. If step 1 is successful, remove the 1st new drive and replace it by the 2nd new drive. If the rebuild is slow, then you know that the 2nd new drive is defective.
EDIT: Unless you already have a complete backup, make sure that you always have at least one RAID member that's working fine.