I am also experiencing this issue with a fully up-to date Ubuntu 12.04. My CPU for compiz at idle is always atleast 16%. When I restart the machine it goes back to sensible levels for a while. I can tell when It goes up again because windows don't move smoothly and my whole system seems laggier and less responsive and applications seem to take slightly longer to run.
I am also experiencing this issue with a fully up-to date Ubuntu 12.04. My CPU for compiz at idle is always atleast 16%. When I restart the machine it goes back to sensible levels for a while. I can tell when It goes up again because windows don't move smoothly and my whole system seems laggier and less responsive and applications seem to take slightly longer to run.