I remember seeing people mentioned this years ago and only got back into the game recently. It's 2022 and I'm wondering, besides the amount of CC which I don't think is the lead cause, has anyone ever dig into the codes to see why accessory/makeup part in CAS slow down the way it does?
It is strange especially other parts of CAS (at least for me) such as clothing & hair have significantly more CC.

I'd think it's partly because those files generate cache and thumbnail files, so you'll usually get slower loading if the cache files get deleted (but if you don't delete them, your game won't always run, so there's that...).

Whenever you load in something new that doesn't have a thumbnail, the game generates a new one, and does this for each item, hence why you often have to wait for each item to load. The same is true for Bodyshop, which has its own thumbnail file (cigen.package). You'll often notice that when Bodyshop or CAS loads something it has already loaded before, the item pops up a lot quicker.

For each run it helps to load CAS, "buy clothes", or "Change appearance" at least once - this speeds up the loading for the next time during that same play session.

Loading thumbnails for the game doesn't help for Bodyshop, and vice versa, since they have different thumbnail files.

That make some sense to me! It is rather strange that even with less amount of CC in accessory/makeup in comparison to clothing in my game, the lag & wait for loading just feel somewhat more severe, which made me think whether their thumbnails generate/behave differently from the clothing's.

All CAS items use (as far as I know) the same thumbnail generation process and files in the TS2 Documents folder, but accessories and hair usually have a lot more ages to generate thumbnails for (and whatever else they're doing behind the scenes) than clothes, while clothes only have one set of resources per file.
Makeup - they usually have custom thumbnails (if small) that need loading in. Usually multi-age, too.
Beards/eyebrows don't usually have custom thumbnails, and not too often custom meshes (rarely high-poly ones), so would probably load faster. Usually multi-age.
Eyes/skin usually don't need custom meshes, but have custom thumbnails. Multi-age, rarely available elsewhere than CAS so not too big of an issue.

The more you have, the longer the loading will take. I think the Accessory.cache file takes stock of every item you have in your game, and if you have the "show CC at startup" button ticked, it will list all your items (it's basically what the game is doing while loading - figuring out what you've put into it).

Why it's necessary that the game spends all that extra time loading CAS and all the CAS-item related menus, and why it spends extra time applying skin overlays and changing between life forms, and all that? Gotta ask the developers about that... Been wondering ever since I started playing. Having a faster computer can help, but still being stuck for several minutes of loading a "Change appearance" menu isn't too fun.