Great insights into your system. I agree, migration from Piecestore to Hashstore sem to be completely random IO seek limited. At least for me, I migrate at around the same speed it took me to simply move a Piecestore from disk to disk in the olden days, with the caveat that since I’m writing to the same disk that I’m reading from, performance tanks (even further lol), once write caches are filled.
This could be reason for uprise and rebellion, but the migration is a once time thing, so I don’t really care.
In a side node, I’ve built a new rack and bigger to house my equipment, but I’d like for the migration process to be finished, before I power down anything. Rack migration will take place after Hashstore migration 
New rack is 28u, following a modified version of Toms plans, that I’ve had bookmarked for what feels like an eternity