I tried this on a few systems, 2 pcs and a laptop in case it initially was my card was going. Same problem, some castle seiges, not all, slow down to almost a halt.
I know its not helpful me not mentioning them specifically but im up to 3 so im going through every scene to find which ones are worst culprits so i dont miss any. For what ever reason when the last enemy dies, speed returns to normal. Then will do a replay through and check each one again in case its some quirk of a save file.
flints finger cant be finished as cant get troops past a block in a doorway. Or spawn of troops where i cant get to.
Twins scene, you have to guide your troops through the gates of each keep as they blob up refusing to go anywhere. feel like im a sheep herding contest when seiging this castle :)