Merkle trees! You (simplified) hash the data at the leaves of the tree, merge adjacent leaves (concatenation for example), and hash the new leaves, until you're left with just the Merkle Root, a single hash representing the entirety of your data. Verifying the root is easier than loading the data itself into memory and hashing it there since you can verify it piecemeal and without loading it all into memory at once.

FTA: "This is the ingenious part: verification carries little overhead, as it runs as a rolling process once the top-level ‘seal’ has been verified. So verification is a continuous process as the SSV is mounted and accessed."

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.