I don’t think there is one single change that made a huge difference. I follow the changelogs posted on the mailing list, and of the performance related changes

I don’t think there is one single change that made a huge difference. I follow the changelogs posted on the mailing list, and of the performance related changes, it’s often “we got 3-5% speed up on this benchmark on this fs without making things worse on others”. Over 8 years and tens of those changes, it adds up to a significant performance improvement.