What kind of mouse is it?. It is a laser mouse or one of those old ones with a little ball in it?
If there was a Driver CD included, reinstall the driver with that CF.
And does the mouse lag in Crossfire only or does it lag always?
It happens all the time or only in games? Try doing what Philong said, if there's a driver for it try reinstalling it and checking on CrossFire, if doesn't work try checking the CF settings.