Settings that impact FPS the most

[Neige]

AMD 7850 here, just notice the huge impact of Unit details setting. The jump from Performance to Max Performance more than doubled my frame rate. The downside is that all the soldiers in each unit will look the same, but for some people that might be worth the trade.

The most taxing settings in my experience are the following:
1. Anti aliasing
2. Unit details
3. Shadows

I have all of these on the lowest settings and I actually get playable frame rates now. Stuff like Texture resolution, Texture filtering, sky, grass, terrain quality can all be cranked up without any noticeable frame rate drop.

[Godz_Mercenary]

This means nothing. Someone with a higher end CPU won't have issues with any of those settings, but could have issues with other settigns if they have a lower end GPU. What's taxing and what's not is dependant on setup.

[A Barbarian]

I would add that for Anti Aliasing, MSAA modes are much heavier on performance compared to MLAA, which is only a post processing filter (which is of lesser quality than MSAA and results in a certain amount of blurring).

Anti Aliasing is heavy on the GPU. While
Shadows are pretty heavy on the CPU (in Total War games).

Other heavy hitters are SSAO and 'screen space reflections'.
Particle effects can be pretty punishing as well. E.g. in a scene like this:

Spoiler Alert, click show to read: 

What's taxing and what's not is dependant on setup.

While that's true to a certain extent (e.g. AMD CPUs have much worse single core performance and loose a lot performance when a setting requires it), some settings are just much more taxing compared to others, in big parts independent of GPU/CPU brand/model.

[booker]

Mi biggest problem on unit detail is that at max performance bodies disappeear.. otherwise i would play with that and everything else on max quality. But i can´t stand a 3200vs3200a nd having 0 boides on the ground.