According to
https://multibit.org/en/help/v0.5/help_importingPrivateKeys.html6. After the import, MultiBit then replays the block chain from the replay date to find the transactions for the new private keys. It is best just to leave MultiBit to sync the block chain on its own.
The replay date being a date you give it. I believe it would update the bloom filters and replay the blockchain from the date you gave it grabbing associated transactions.
That sounds expensive in terms of node I/O. The Bitcoin node needs to rescan the whole blockchain (if the date you give it is old enough) looking for transactions relating to the address you give it. Is it possible to DoS a node in that way? It seems like you're getting the node to do a whole lot of work by sending it a very simple request.