i am playing Huge board with 6 npc opponents each opponent have at least 25 cities. There is no lag but the game crashes so after 2 or 3 turns from my save. I reduce the shadow that helped some but always the same. I do not know if the game can handle large kingdoms. 

  I have the game set for rarely quit or surrender but some of the NPC kingdoms out and out quit the game with 25+ cities. They just leave the world.

I do not think the game know how to upgrade their troops. Right now my knights and mages are super OP and can kill enemy units in one hit. I move my units across the board with no fear they cant handle anything they come across and monster groups do not even approach them.

The AI does not forget past relations. I have tried to make other nations like me better buy giving them resources but to no avail they still do not like me making diplomatic victory impossible.

The game as a few quirk but I still enjoy playing it.