the game spawns in more characters than it strictly has to and doesnt clean them up quick enough. when you unland a character, their entire court is flung to the other side of the map and when you land a new person in their place, a bevy of lowborns is created to act as courtiers instead of reusing those wandering characters that already exist. events are constantly generating characters that dont get pruned when you dont recruit them, and even when useless characters are killed, they clog up the save as corpses for far longer than they need to. many events that generate characters also lack a cooldown, so not only are they generating useless characters that impact performance, theyre also common enough that you get sick of seeing them constantly (hedge knight travel event comes to mind)
there is a lot you could fix in this game that would help with performance without arbitrarily dividing the map. we could have all of china and have it running smooth as butter, theyd just have to clean a bunch of shit up first