I'm guessing your array is slow because it is rebuilding. If you check mdadm -D /dev/arrayname, you can wait until it's rebuilt and clean, then check the speed again.
On the other hand, it could be something else. I found this https://bugs.launchpad.net/ubuntu/+s...id/+bug/449876
which sounds a bit similar if you've used "fakeraid" and dmraid in the past. Basically, you need to use -nodmraid as a kernel option.
On the other point, when you update, I believe you have to update your initrd manually. Did you run something like?
mkinitrd -o /boot/initrd.img-2.6.26-2-amd64 2.6.26-2-amd64