^ it's mostly the sky box generation in my experience. So if you change the details on that you'll change the jump time. (I often go way below minimum when I do a long trek)
It's loading the textures and objects for the star system you're about to jump to. It takes a LOT longer in Open or PG's when there are a lot of player ships in that system too.
There is also the calculation for the state of the system that governs NPC spawns and poi, network handshakes to see if there are any 'islands' with other players and suchlike.
Yep, all of the above. The amount of processing going on under the hood (both client and server) is pretty impressive. For the skybox alone they have to work out the intensity, colour and position of every star in a given radius around the destination system, then apply dust clouds, nebulae etc. I'm actually amazed it works as well as it does.
Hyperspace transition was actually sped up by a second or so in a recent update, by changing the way stars are rendered on arrival and sacrificing some initial LOD quality in favour of speed. It made some dwarf stars look terrible, and many players called for the old
longer transitions to be reinstated. Not everyone feels the same way about the delay.
I do feel sorry for anyone who genuinely can't stand the "loading screens", because they are a core part of how the game works and there's really nothing that can be done to speed them up other than maybe pre-generating some data during the FSD charge. But I suspect the benefit would be marginal at best.
And entering glide or dropping out at stations has improved dramatically for me with recent updates, to the point where in Solo it's instantaneous more times than not. In fact if I wasn't aware of how instancing works I'd probably assume the Solo transition was an aesthetic choice rather than something hiding an instance change. It can still be a little sluggish in PG and Open, but that's mostly P2P networking and I doubt there's much FD can do when it comes to the quality of everyone else's connection.
It certainly feels more "seamless" than in any earlier iteration, across all modes.