Like Cicero says. Unfortunately, many of our prior cities were not made with performance in mind. This is for many reasons.

1. Making a city is hard -- making one while taking FPS into account is harder.
2. When an exterior maker builds a city, the FPS hit is not necessarily apparent prior to all the NPCs and scripts being added, by which time its usually too late.
3. The factors that most affect MW framerate were not widely known until relatively recently (collision shapes, drawables count).
4. For many years, there was an expectation that newer hardware will solve all ills (it didn't), so these issues didn't receive as much attention as they should have.
5. Many of our cities were built over the course of decades -- rarely does a developer have the opportunity to start a city from scratch with FPS in mind.

Mind you, many people like the cramped nature of Old Ebonheart and few are campaigning for a total redesign (aside from Cicero). A lot can be done just be making bespoke assets and using them in a smarter way. New games cannot rely only on raw computing power to cram detail in their scenes. Smartly made assets (something which vanilla MW suffers a particular lack of), smart engine features, and knowledgable use is how its done.

Roa Dyr is a particularly bad case of a lack of reasonable assets. The result looks great overall but the limitations of the Mournhold tileset really show up close. The FPS issues are also due to the excessive use of assets in non-intended ways. Gnomey's idea was always that the current technical solution is temporary and will get overhauled once we have better assets (while keeping the design relatively similar).

But yes, we aim to do much better in the future, as Chef has with Narsis. By looks, that should also be the case in Kragenmoor.

As for advice, I can only advise reducing viewing distance, and turning off shadows and post-processing. Better the game look slightly worse than have non-playable FPS.