There's over twice as much content in the first test. It's around 3.8gb/s vs 30gb/s if you divide both folder size and both du durations. That makes it 7.9 times faster and I'm comfortable calling this "almost 10 times".
oops. I missed the size diff. that's a solid 8x. that's cool!
The total size isn't what matters in this case but rather the total number of files/directories that need to be traversed (and their file sizes summed).