Found the issue for this slowness, in the RAID rebuilding it had not moved but 1 % in the last 1 hour, I started to shut down (properly) to do a reboot, and once it was down i decided I will keep it down 10 min and then start it, and it moved 3% so far in the last 5 min, so it had hit a particlarly slow spot in the rebuild. I plan to keep it down a few minutes more then turn back on, and raid build should continue in background with minor impact only. It may have another slow spot later, but I hope not.