Hello Anvi,
I have open a case to italian support. The reply was that the speed of rebuild is depending from my pc (cpu or network)........ this reply is wrong because the rebuild is made inside the dns also if I'm not connected. I think there is a bug and the firmware must be improved, but dlink does't want to invest on this product (last official version of firmware is very old).
I don't want to give up to media library....I'll try to open a case to uk or us support.
Marco