Firstly, all bodies instantly return the detailed body info (Composition, resources, volcanism, etc.). This is the same info which used to be provided by the DSS. This info appears to all be stuff that would be coming out of the system formation, so given that plus the instant return of the info is presumably generated in hyperspace.
Secondly, rendering the planet images in the FSS also takes a similar time for all bodies. (I've timed it multiple times.)
Then, with regard to the body types:
A.
Non-landables - Planet Generation system Sims them up, but no surface POIs, so no delay other than in image rendering.
Image of a Non-landable transitioning between 2 levels of render detail below. Note all old DSS info, plus Locations has already been returned.:
B.
Landables without volcanism - Planet Generation system Sims them up, POIs return just before the point that the visuals render to the level of the final FSS view. Images below showing that point. Note the first image isn't the first level of rendering, and also the 3 second time gap between the images.
C.
Landables with Volcanism - Planet Generation system Sims them up, visuals render in a similar time to other types, POI results take a long time to return.
It would seem that for a body with volcanism the full run through of the planet generation takes longer than for other bodies. This would make sense as simming up a planet with active geology should be more involved than simming up a geologically inactive planet.