[dredmorbius]

To be fair: the case described here is two magnitudes larger in both directions than your use case.

And the critical aspect from a performance perspective was where the hash table became too large to fit into memory. Performance of all sorts goes pear-shaped when that happens.

rsync is pretty incredible, but, well, quantity has a quality all its own, and the scale involved here (plus the possible in-process disk failure) likely wasn't helping much.