Dropbox — Large Sync Operation after Windows 10 Feature Update
After Windows 10 installs a Feature Update, Dropbox may trigger a large sync operation:
This is primarily a local database operation and Dropbox is not necessarily downloading all the files again.
Just leave it running and it will settle in its own time. You can continue working in the meantime.
For large collections of files, let the machine run overnight. The Dropbox icon will appear idle when complete: