Answer
Synchronize rebuilds the Oracle Text search index's actual content — picking up new, changed, or removed documents since the index was last synchronized; conceptually this is a business-level 'rebuild what the index knows about' operation, and its runtime scales with how much underlying data has changed since the last sync, which is why it can take far longer than expected on an environment that hasn't been kept in sync. Optimize instead reorganizes the existing index's on-disk storage without picking up new content changes, conceptually similar to defragmenting a disk, and is a technical/housekeeping operation rather than a content-refreshing one. Running Optimize doesn't substitute for regularly running Synchronize, since only Synchronize actually reflects new or changed documents into the index's searchable content.