Well if you use aws s3 sync, then know that this in fact relies only on timestamp and file size to compare files. If there's a difference it'll send the whole thing.

rsync is smarter it actually can compare and send parts of modified files. This article talks about new functionality which utilizes zfs snapshots. Now zfs knows better what has changed (for example whether file was renamed or moved) so it can be even faster when replicating the changes.