I'm a bit confused as the bug does not seem to appear often anymore...I've been playing with and without mods, it lagged both times, I am very sure of that(around 100hours before I even knew there already were mods). After a graphics driver update, the problem is not as persisting as it was(not constant anymore), but still there - it's also a lot less stuttery as it was before. And I've yet to experience it with mods.
Also, but I'm very sure you're aware of this: Something about the AI Path makes sieges freeze and the game barely playable.