If you had BTRFS – you could simply do btrfs send | btrfs receive. The slowness is because you were attempting to copy file by file. Storj creates a lot of small files, so most of the time of course will be spent seeking, even if you have metadata cached. .
Sending filesystem will just stream everything at max sequential speed.
big mistake.
Oh god…
Read about btrfs send/btrfs receive, and review this: Zfs: moving storagenode to another pool, fast, and with no downtime. It is about ZFS, but with btrfs you can do the same thing almost verbatim. Otherwise why are you using btrfs if you are ignoring all its features and advantages, might as well use ext4 then:)