Indeed. There would be better gains with btrfs, actually—around 3% compared to ext4 defaults—almost twice that! But…
…it was slow as hell. I’ve tested several scenarios. Regarding the original post, except for the -I 128 switch reducing time of du (and hence likely to run the filewalker process), I’ve observed no changes to default ext4 with regards to performance.
btrfs was more than twice slower under all metrics even under the best settings I found (mkfs.btrfs -m single -d single, mount -o nodatacow,noatime,max_inline=0). I suspect that bandwidth.db updates dominate I/O performance there, maybe moving them to flat files—like the orders were moved—would help. But at this point this is only a suspicion and I’m waiting for some other tests to finish to check this as well.
Pretty much just that the inodes are closer together (being smaller and having less of them).