Hmm, can you try running iotop, to see how much is being red from disk? Is it 20Gb or 6Gb? Maybe the prefetch is able to fill in before page faults are happen

Hmm, can you try running iotop, to see how much is being red from disk? Is it 20Gb or 6Gb? Maybe the prefetch is able to fill in before page faults are happening? Or maybe you are hitting the disk cache?