Re: Strange in-battle FPS drops after upgrading PC.
The wall destroyed interlude is a 'cut scene' playing, like 'general dying' - you can disable that in the configuration file (medieval2.preference - open with Notepad):
[game]
event_cutscenes = 0
Set the value to 1 and then make the file read-only as the game will otherwise revert the change (not really advisable) - this applies only to the Grand Campaign, if you play mods then add that entry to the mod's own configuration file.
The cut scenes are in the same format as the intro movies if I am not mistaken - have you got a FPS drop when they play?