AO is supposed to be used for masking (I.e. dirt) only, not as a part of the scene's shading. Furthermore it is very naive, as it produces just grey gradients, by doing per pixel sampling. The higher the radius, the simpler the geometry, the more it will seem, that noise never goes away, as you enter the territory of diminishing returns.
It is not part of the main rendered scene and is a separate pass, that is being computed seperatly.
It has two quality settings. One in the material and the correlation to AAvsGI, as in how many rays are cast in total per pass.
The one in the material sets how many rays per pass compared to other rays, like GI, are being cast.
The one in the material is the one you should increase to get better quality, at the cost of the rest of the render being rendered more slowly, as AO "steals" processing power from the "rest" of the image.
As far as I recall, Default AO is pretty low on the rays being cast side, as such you have to take many passes, to get a smooth result. If you do many passes and low rays, you "waste" processing power on sampling (AA).
But this is usually none of the users concern, as the engine is supposed to regulate that, but since we are in a seperatly computed pass, the render engine does not auto adjust via adaptivity and we have to get dirty.
The default assumes, that you have low radius and use it as a mask for materials. If you have different needs, like higher radius or extra compositing in Post, then you have to increase quality.
Here is a tutorial, that shows how it is supposed to be used, in its default state.
