Slow sync
All records on a device are examined for modifications, additions, and deletions. The records are compared to a cached checksum to figure out if the record is different. This takes more time than a fast sync.
Fast sync
A device gives iSync a list of additions, modifications, and deletions. Because records on the device aren't looked at individually, the sync is faster.
Important: Information about products not manufactured by Apple is provided for information purposes only, and does not constitute Apple's recommendation or endorsement. Please contact the vendor for additional information.