Let me share something here, in case it helps.
There´s still at least one old issue that persists nowadays and may explain what happens here, but only Asobo can confirm if it´s really the case or is related to the performance drops. The issue is the following can be easily noticed during flying if you pay attention to screen: autogen area (generic buildings) just outside airports boundaries gets refreshed for some reason when you are close to airport (around 5nm away). This means autogen is rendered once (when distant scenery is loaded for the first time) and at least it´s rendered for a second time when you are approaching airport. This happens even on generic airports, not only on handcrafted ones. I reported something similar by 2022, related to custom photogrametry areas, but I could check later on that the autogen issue was also reproducing the same behaviour.
This is not apparently an object LOD change but a complete regeneration of buildings in the airport surroundings. I say that because new building types will be displayed which may not be exactly the same ones which were existing before. Only in case airport includes custom buildings then same ones will be respawned, but are also respawned. This can be seen in the case of my orginal bug report. You can see how they fade out and fade in at my report screenshots. Do you remember the old autogen trees in the early years? They were fading in and that was changed as people complaint. Buildings are now doing the same.
That second refresh should not happen, to start with, as scenery was already loaded and rendered. On the other hand we have seen at OP´s video on the following topic Help trying to diagnose stutters how a distant city spawning can produce a 1.6s freeze when that happens in one go. If this autogen refreshing is not only happening that second extra time but is happending more times (in between the first time cities next to airports are visible and the time you actually arrive to airport) and even more, it´s happening for all airports nearby, it would mean that scenery refreshing is being called multiple times without any real need to do it, and that´s adding a significant extra load to threads.
This is just a speculation as I can´t see the code itself but the fact is that even nowadays I don´t find any technical reason that expalins why the original extra refresh alone (the old issue) still has to happen, so maybe a bug could exist and is asking to refresh scenery multiple times even if not required at all.
Cheers