Simply bad framerates in places like Daggerfall/Wayrest castle and Scourg Barrow, haven't got too deep into the game so those are the only ones I've been in. I'm fairly certain its just because my PC is an ultra potato and I shouldn't be playing anything more modern than Plants Vs Zombies on it, but still felt I should at least mention it and post my god awful specs.

PC: HP p2-1119
CPU: AMD E-450 APU with Radeon HD Graphics 1.65 GHz
RAM: (5) 4.6GB
Windows 7 64 bit

I'm not aware if DF or DF unity have any level of Occlusion Culling but if they don't that could be a good start for trying to optimize big dungeons, doesn't help to render the whole dungeon when I look at a wall. Its something I've noticed tends to not be implemented in general in alot of modern games because of a general misconception that anyone who games has a gaming ready pc.