In an earlier Version of bitcoin-qt the line was the following:
SetBestChain: new best=0000000000000419f719 height=212580 work=666804971104474944731
In the log messages, new best is the start of the hash of the block
that it considers to be the head of the best chain, height is the
length of the path through the tree, and work is the sum of the work
embedded in that path.
It basically means that your client received a new best blockchain, so a new block was found. You can compare those log entries, they will allways increase in height and its always followed by
I'm not sure why this is CPU intense for you, i didn't recognized this at my client yet. There is no way to remove this function because its a fundamental one of bitcoin-qt client and the bitcoin protocol. Also making it less CPU intense can only be done by changing the source of the client.