Okay Phillip, give me what you got! I think the only valid evidence is using people other than the main heroes of the stories. Cause, we know that it would be hard to adjust when JY needs to show us great progress in a very short time. Basically, it would be hard to adjust.So my reasons for thinking it doesn't slow down do to mastery is because:
-GJ was at least 3 times slower than he was in LOCh during the 13 year period. Lets say GJ's deterioration is proportionally less since he used 9 Yin, the most advance type of cultivation in LOCH and ROCH. So if we say that the Greats is at 18-19 year old YG's level at 30, then they would never have reached their level in LOCH and ROCH is they improved 4 times or more slower than before.
-HR and YD also knew 9 Yin, and should have long mastered it after so many decades. But since they did not imrpove even close to the rate of GJ with 9 Yin, doesn't it makes the slowing(if there is any intended by JY) insignificant as long as you train?
-A martial artist that cultivated for a long time can almost never improve dramatically anymore even in a decade of period if they slowed down like GJ and YG did. So it is most likely it's JY's bad adjustment mistake.
-There is no trigger to it because people improves at different rates. So "amount" isn't a trigger. Say QQJ compare to ZBT. QQJ can never be in his league even when they both mastered QZ cultivation so long already. If people really do slow down, then could diciples gain up to their teacher's level really fast? Cause, a newbie gain a lot fast, while the teachers already slowed down exponentially. So how come in most sect like Shaolin, QZ, Wudang, etc., teachers are mostly way superior?
-Lastly, it's illogical cause your body has virtually no limit in the capacity of Chi you can have.