They did this by overclocking the Switch during load. I didn't see any mention of boost/overclock in the Factorio writeup, though.
I found a GameFaqs forum thread[1] from shortly after the game launched that claims it was about 20 seconds. And a US Gamer article[2] that says the update that came a couple years later only shaved off about 5 seconds (seems within the ballpark of what I measured)
[1] https://gamefaqs.gamespot.com/boards/189707-the-legend-of-ze...
[2] https://www.usgamer.net/articles/breath-of-the-wild-load-tim...
It overclocks the CPU at the expense of GPU. It's suitable only for loading screens or other areas of the game where you don't need to render at more than single-digit FPS, but can produce a 20-50% improvement in loading time.
Perhaps they don't get to mess with the clock "willy nilly" and it's more like an API that enables a higher - but fixed - clock speed?
[1] https://www.shacknews.com/article/112895/crash-team-racing-n...
