Sub-20 was clearly the hardest for me. sub-1 -> sub-25 was a quasi-linear progression. Then, I stucked at 21 avg. Once I broke the sub-20 barrier, the progression from 19 to 13 was quite fast. I'm now trying to break the 11 barrier which is quite difficult for now, but it'll fall soon!
I'm pretty sure there is a psychological effect when we are close to a barrier. I was stuck at 21 for like 2 months, and when I broke the sub-20 barrier, I went down to 17ish in like 2 weeks. I guess we could see the same kind of effect with the world record. It stays about 10.xx for a while, then Feliks arrived (on his majestic white unicorn). Right after him, dozens of people broke the sub-10 avg...