Performance Tip #9825 - Add indexes & schedule a job to run

Examine indexes in the AsyncWorkItems tables

It is important to be careful with the AsyncWorkItems and AsyncWorkItemStatusHistory measure group tables. These tables are used to queue the jobs that are processed by the Planning workflow engine and therefore will have frequent insertions and deletions being done. Because of this, these two tables experience rapid and severe index fragmentation. Your database administrator must monitor the fragmentation percentage closely and rebuild the indexes more frequently than the other tables.

http://technet.microsoft.com/en-us/library/bb794627.aspx

How to automate index maintenance
The following script checks for all indexes on a Planning Server database and finds those that have an average fragmentation more than 10 percent. Then, depending on the degree of fragmentation, it performs a reorganize index or a rebuild index for those indexes.

http://technet.microsoft.com/en-us/library/bb838727.aspx