It's not a deal breaker for me - I just left it running for a couple of hours, but the process seems sufficiently fragile that many people had quite serious problems getting it to download.
I'm not a game dev, but it feels to me like there's a few areas where FS falls short of what I'd be expecting as Microsoft. I'd want it to be responsive and start quickly (at least presenting something on the screen), and I wouldn't want it to be using full GPU in the menus. Teething issues are to be expected, but the downloader is critical and it's the first experience people have. A shame they didn't use some more reliable file transfer tech (or even just thread it!) The general lack of multi core use on the sim is definitely quite disappointing though for a 2020 release that's doing lots of general calculations and other tasks that benefit from threading.
The issue is, as you say, a single-threaded "download-extract-next" flow... It's 2020 and that's what we'd expect from a 90s game launcher. Steam and other game launchers/downloaders can all saturate an arbitrarily fast network connection. Heck, even Windows Update (BITS?) can do this well. A shame Asobo wasn't able to leverage any of this.
The amount of GPU is indeed strange - I think there's more than meets the eye here. There seems to be a lot of HTML/JS type stuff in the UI here, and I wonder if they are doing some kind of GPU-based full-screen rendered surface, rather than just having a simple oldskool Windows Forms app that does the downloading without GPU acceleration.
Ugh, yeah as a VR flight sim enthusiast this is just sad. Essentially every semi-realistic flight sim on the market is main-thread bound even on high-end on modern CPUs. I was really, really hoping that MSFS2020 would finally be the modern engine sim we've been waiting for so smooth VR flight could be enjoyed.
Quite strange, as in the tech world we normally super optimise the initial onboarding user experience. Especially when many will try FS through Gamepass, it seems strange to not focus on the "onboarding ramp" to try and convert and retain users.