Don’t these machines have read speeds of like 5000 MB/s? So just read the 9 GB blockwise and apply a super simple hash. Should be done in two seconds.
That's uncached, so it's not that quick at boot. 9GB would take ~19s then. Block would go faster, but not an order of magnitude faster.
It's a Merkle tree, apparently, so parallelizing the checksum should be trivial.