I'll add my weirdness observations to the conversation. I believe it is related to the 0 HP bug others are reporting. If I cast call of titans every unit whos ever died is also summoned at 0 hit points. Even if it was no longer visible on the map (ie it may have appeared to be destroyed correctly as it was no longer on the map). These destroyed units are not being cleaned up correctly. They still can be re-summoned after loading up the auto saves. So their data is being saved. These uncleaned up units are basically a memory leak. I'll see about posting the saves later.
Another thing I noticed. I'm currently playing as kingdom. The land doesn't change to green fields when I take an enemy city. I dunno if its supposed to. But if I load an auto save, suddenly all my territory is switch over to nice green fields. I dunno if its slowly supposed to spread or what. It is definitely not working as intended. Against the new AI (on Hard) it makes a difference because they are running around with lord hammers and things. A +20% net swing against an empire civ is huge.
AI is sluggish late game (turn 400) as you've mentioned. Can become unresponsive for 20 seconds or so between end turn clicks.
Now about my current game (medium map, world HARD, 9 AI's on HARD):
I lost my capitol to some wandering monsters early in the game.
I had to rebuild and try to catch up to the AI.
UMBER built several towns around me and basically walled me in with their ZOC.
I didn't feel like paying them off so Umber and Yith declared war on me.
I finally gained traction and took a city from Umber. Stupid me, now Yith had a way to get to me.
Yith took the city from me. I had slightly better troops but he had lots of groups of units. On home turf I wipe the floor with him. When I invade the 20% swing to attack/defense means he wipes the floor with me.
My one city wasn't cutting it and I had not metal. I spent some time in the adventuring techs getting all the resource spawns. Luckily all of them appeared in my zone of control.
This game is taking forever with the turns taking so long due to the AI delay.
With my additional resources and maxed out unit tech I slowly start pushing out.
Even with my Sov decked out in Legendary armor and maxed weapons, he can't auto win battles against AI armies. Normally at this point you could steam roll everyone with your Sov. When their guys all have lord hammers and you fight on their home turf they will win auto-calced battles if you don't bring enough fodder along. You can will in tactical if you can get your Sov on a +10% def square and use spells. Perhaps Kingdom factions I could walk all over but I haven't seen any yet.
I've now crushed Umber, Yith. An with Umbers three additional iron mines I can pump out heavy units at a decent rate.
I'm slowly rolling across the map, the slowness with the AI is the most irritating thing the turns are letting me click a little faster now that a few factions have bit it.