What exactly does the render quality setting change besides craft shadows?

[cosekantphi]

I play KSP with the render quality set to fastest because each render quality step comes with a massive performance penalty. Setting the game to simple, I can expect to go from 40fps down to 15 or 20 in atmosphere with a modestly sized craft of about 50 or so parts.

Unfortunately, the stock game limits shadows to simple or above; the fast and fastest settings completely disable them. Not that this is a reason to tolerate the fps loss, of course. While shadows exist on simple, they look like pixelated trash even with shadow cascades set to maximum. So as far as I can tell, shadows seem to be the only thing the render quality settings change. Literally everything else looks the same to me. Naturally, I had assumed for years that this meant shadows were  just too much of a graphically intensive task for the non-gaming laptop I play KSP on.

But then I tried out parallax on a whim, and it was nothing short of a miracle. Somehow parallax allows for decent shadows to exist even with the render quality set to fastest, and somehow it does it without a noticeable drop in performance (discounting the main features).  Even if said main features become too much for my laptop to handle, I can disable them by setting the terrain shader quality to low. This still retains the craft shadows.

Is parallax doing some kind of magical optimization here or did it just untie shadows from the render quality setting? In case of the latter, what exactly is happening on the higher render quality settings to eat so many frames? Because it's clearly not the shadows. Texture resolution and anti-aliasing are tied to different settings, so what might be left?

Edited April 11, 2023 by cosekantphi