Do you seldom encounter slow mapping with flashbackup?
Look at your exclude file list. Do you really want to exclude 50% of the drive? When performing Flashbackup, setting up an exclude list will decrease the performance of the VxMS mapping. The exclude list processing requires a lot of resource and time, so excluding too much data will slow down performance.for an example if you are backing up a 1TB drive using FBU, mapping should take around 35-40 minutes in usual scenario but if you have a exclude list of say about half of TB, It may take a whooping 7-8 hours in just mapping.
Finding the fish in a large pond with millions of fish and mark it not to be fished. Not a great Idea and it need too much work to do. My advice on this, not to have a large exclude list while using FBU. your Back-ups may not work as fast as it use to.