Why does the realm menu lag so much?

[scaper12123]

So I'm doing a special run for a friend who wanted to see if it's possible to take over the whole world in CK3. So I made an immortal ruler, gave them absolutely bloated stats, and set out. But by 1200, I'm finding the game is slowing down. Par for the course in a Paradox game, but the biggest offender is the Realm menu. I click on that and I have to wait for 10 minutes for the game to unfreeze and show the menu. I've not even taken over half the world yet, and I'm still under the base maximum vassal count for empires!

What the hell is wrong with this thing? Is it trying to look up every single township in Africa and Western Europe and decide which ones it needs to add to my list of realms or something equally non-optimal and stupid? And I guess so long as you're here, got any mods that can cut down the lag that you can share?