Rclone can "sync" with a range of different ways to check if the existing files are the same. If no hashes are available (e.g. WebDAV) I think you can set it to check by timestamp (with a tolerance) and size.

Edit: oh I see, delta transfer only sends the changed parts of files?

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newsoftheday 6 hours ago | parent [-]

It only sends the changed parts of files (the diffs) is my understanding which saves bandwidth.

	▲	mnw21cam 24 minutes ago \| parent [-]
		However this is only really good for slow connections. If your connection is faster than about 50MB/s then the delta calculation mechanism becomes the bottleneck. On fast connections you should use the -W option for rsync which switches the delta algorithm off.