Hi,
Synchronization always take place incrementally but not the first time. In other words the first time it has to sync everything after that it only checks the changes that have been made. If you don't use previous version one tip would be to turn off page versioning for your book that is synced on OneDrive. PageVersioning creates a shadow copy if you want to go back to previous versions. You can imagine if you use video this will also be on the previous page versions. For pageversioning look under the tab "History".

Maurice