I have a Thecus N2100, and while it's working flawlessly, it's
*really* slow with disk access, like 5–10 times slower than the
hosts that I want to sync with.

This made me think that I ought to be able to tell unison that this
replica (let's call it A) is really slow. That would have the effect
that if e.g. A has a new file not present on B, the file would be
transferred to B and only then would its checksum be calculated and
stored.

Also, on inititalisation, it would be good to be able to say: "I
know A and B are identical, so please just calculate the checksums
on B and believe me when I say they are the same on A; don't check
it."

Thanks,