I believe adding will have to rescan the blocks otherwise it wont know how many funds are in the address I think.
Not a good idea to add 10000 addresses using importprivkey when each import triggers a full rescan.
At least with pywallet i could add the addresses and then bitcoin core could rescan when i start it, but not for each import, it would take a week.
Is there anyway to add a private key without trigging the rescan? What if i know that the address has not been used before and i dont care about stupid rescan?