Problem is as stated in the title.
Since the last patch on 09/04/2024, models in-game tend to flicker, sometimes white, sometimes in their own colours. This applies to NPCs, Horses, Players, certain outfit parts, ships, etc.
It is not an issue on my side, and everything that you might suggest to try and fix it has already been tried, including a full reinstall of the game.
It seems to predominantly happen in cities, where lots of models for all kinds of things are present.

Secondly, the game gets even more sluggish after having it open for a while than usual.
Now, this issue has been present in this game for years by now, but it has gotten a lot worse with the last patch. The game starts off performing good after a fresh start, however, after a while of it being open, its performance gradually decreases. Since the last patch it is so bad, infact, that I lose up to 80fps in certain areas, standing at the same spot, same camera angle, same settings, etc., with the only difference being that the first measurement was taken after starting the game and the second measurement being taken after traversing the world for a while and coming back to that same spot.

I am not optimistic about this ever getting fixed, since fixing the game's performance doesn't yield a quick buck and specifically the second issue I presented has been present and well-known for years now.