Synthetic Backups

I am trying to implement a synthetic backup on a client that over a TB in size, and has about 3 million files.   The  files do not have a high change rate, so this m,akes this client a good candidate for synthetic full backups.   I copied my production policy and cretaed to a new policy (and selected Collect True Image Restore Information with move detection)).   I made a one time full schedule, a daily incremental schedule, and a synthetic full schedule.   I ran the one time full backup and it lasted about as long as previous fulls.   However, when I run the incremental, it takes an extremely long time.   So much so, that it times out.   I ran an incremental  from my production policy, and it lasted 15 minutes.   Anyone have any ideas on  what why this