@chrisplyler So here's proof that one should verify what one was told 😉
I tested what I said on an assembly imported from a STEP file :

This can be exported from other CAD systems as an assembly and when imported into Fusion 360 it is imported as an assembly with components.
But I also exported it from as a part and in that case Fusion 360 imports it as bodies in a flat structure.
Then I. exported it as a . f3d once with the timeline enabled and once without, males 4 files with different file sizes.
With components and no timeline 1.1 MB
With bodies only and no timeline 2.6MB
With components and with timeline 2.4MB
With bodies only and with timeline 5.5MB
While simply enabling the timeline more than doubles the file size, using bodies instead of components - at least in terms of file size - also clearly has a detrimental effect.
If that also results in detrimental effects when actually working in an assembly I cannot say at this time.
@Phil.E @schneik-adsk would you have any comments on the performance aspect of this ?