I am waiting about 30 mins for core to import a privkey, why is that ?
I use blockchain.info, and importing private keys are instant. Perhaps your wallet wasn't synced?
Blockchain.info is always up-to-date, that's why importing a key there happens within seconds. It shouldn't take too long when importing into Bitcoin Core either because it downloads the whole blockchain. If you import a key into a different client like MultiBit, then the client has to scan the blockchain for transactions associated to this key as knightdk already mentioned. Keep in mind, even though MultiBit doesn't need to download the whole Blockchain, it still needs to know about the transactions associated to your key.
What you can do is to import the key into the Blockchain.info web wallet and send all the funds to a new key. But then you will have to wait for the confirmations of course.