But if I understand correctly, "power leasing" merges leased power, so it's even easier, as you only need to generate one block in NXT, right?
Yes. But difference between generating with 1 or 1 million accounts is just a few second work for CPU. How fast is harvesting? Not much slower I think?
IIRC, signing (in java) is <300 microseconds (depends on hardware ofc) , so 1M accounts = 300 seconds, that's bit too much (network could be much further already...)
OFC, you could go with C implementation.
but 10_000 accounts should be doable.