Excuse me, but some time ago (before Horizons (?) and after the Wings (for that I am sure), system map was almost instant (always)...
Wild guess, the problem is somehow related to planetary landing informations on the map...
Nope... I traveled to the core during 1.2/1.3 and the map was always quite slow to open.
During our current travels with the Distant Worlds expedition, it has become obvious that the complexity of the system and the resolution of the PC's display have a significant impact on the speed of the system map opening.
I run 4k at 3840x2160 and the system map can be very slow, depending on the number of objects, and more importantly, the number of stars in the system. (it seems that the pre-rendering of the stars takes a long time...)
In summary, it appears to be a graphic processing issue, not bandwidth.