As amaclin and Dabs on this thread already said, you need to add a third parameter false to importprivkey
bitcoin-cli importprivkey privatekey-1 somelabel false
bitcoin-cli importprivkey privatekey-2 somelabel false
Works fine even for 2000 addresses. However, the next rescan will take some time, especially if there were many transactions.
how much rescan time we can expect for 1000 private keys imported to wallet with parameter false to importprivkey command
The import takes about a minute, but there is no rescan triggered.
If you need the accurate balance, you have to rescan manually afterwards (it may be enough to import a private key with third parameter true, but I'm not sure). With my 2000 weak keys (some of them had a lot of transactions) it took a full day to rescan the wallet. My wallet.dat is about 500 MB. If your keys are fresh and haven't occured on the blockchain, there is no need to rescan.
So, anything available in those addresses will be sent ? How do I send partial amount ? Moreover, is not there any parameter to mention the mining fee ? Tx without mining fee may take a long to confirm !!!
No, the commands send no transactions, they just put the private/public key into the wallet. The rescan will just search for transactions and recompute the balance. I don't think that bitcoind has support for sweeping an address. You can of course just ask for the balance and then send that amount the usual way to a new address, e.g. using bitcoin-qt.