I'm going to guess if you did it on the video card and simply disabled those enhancements it would be considerably more energy efficient than doing it on the CPU as well which (a) means your laptop battery lasts longer on a charge, and (b) also opens up the possibility of not needing a particularly powerful CPU in order to juice the resolution, because the video hardware is doing the work for you.

From a programming perspective I do completely get the desire to have the CPU do all the work though - if you want absolutely everything under your control it's easier, and especially if your game is single-threaded.

Its probably not correct to state this. Its probably a combination of how nanite itself is doing Lob, lightning etc.

A mix of software and gpu

At this point words get blurry. Is using CUDA or a compute shader "software rendering" or not?