[kyotov]

I can see your point. We really need to run some experiments in S3 to satisfy this point rigorously.

Intuitively, the more similar the files are, the more 3x-10x+ will be representative in the real world. As the files become more and more different, one is of course at the mercy of the bandwidth between the computers. If you need to transfer 1 GiB differences over 1 MiB/s connection, it will take ~1000 seconds -- no magical way around it.

The comparison so far is practically on the computational cost of the sync, which can be significant as differences pile up.