It's kinda late, so I'm not going to do all the necessary bibliographical work required to get this idea out, but it made sense (makes sense) to me.
Take modern sports. To go from scrub to mediocre is a very fast improvement. For a genius, to get from scrub to master is a very short process (although not for normal people). To see this most clearly, take games like Chess, or instruments like Violin/Piano, and martial arts (where the best martial artists peak at extremely young ages). This only applies to "pure" form martial arts, and not MMA, which is a whole different ballpark. In other words, a couple things are clear:
1. The absolute gap between #1 and #1000 is not huge
2. The speed that a prodigy can go from newb to "high end" is very fast
3. The speed to improve from #1000 to #1 can be very slow
4. The plateau effect occurs immediately upon entry into the upper echelon
I think we can observe this quite clearly in most novels, but there are outliers and problems comparing between novels.
As was stated before, GETTING to "near Great Level" in LOCH/ROCH appears to be "easy." It doesn't take long (within 5 years on the outside). On the other hand, SURPASSING this level apparently takes a huge amount of effort. After OYF was wrecked by WCY, he still caught up to the other Greats, who weren't exactly slacking. ZBT was also able to catch up once he was forced to study MA.
We see similar phenomena in DGSD, where (ARGUABLY) there is a massive collection of fighters at "around" QF's level:
DY/XYP 3/XZ/MRB/XYS/JMZ and at a slightly lower level (but not so much that the upper tier can just thrash them at whim) YTZ, MRF, DZM, etc.
Some of these fighters ascended the ranks at an extremely rapid clip. But, for some reason or another (novel issues?), CLEAR TEXTUAL SUPERIORITY was never shown. Despite, for instance, XZ having anywhere between 3-9X the inner power of QF.
And we see similar things in other novels: HSDS, ZSF and ZWJ (ZWJ reached the top at the beginning, and never improved after), XAJH (everyone but DFBB, who's a clear outlier).
The biggest textual outlier is XKX, which I think is out of the JY continuity to begin with. Obviously stuff like DGQB and Sweeper monk are in the "legendary" category, and can be ignored following general patterns.