Ok, here's another example of the MoUSO issue. Core i7-3520M, 2.9GHz-3.6GHz. I'm installing a new Win 10 21H1. I just didn't the 2021-08 Cumulative update and the button for Restart Now is visible. I minimize the Settings window, and when I restore the window MoUSO uses 50% CPU for 24 seconds, I timed it. Every time I restore it, 24 seconds of 50% CPU. And this is on a 3rd gen Core i7 which runs at 3.44GHz for that duration.
I just rebooted, and the update is installed. I open the Windows Update page, and now when I minimize the page and restore it for seconds the 50% CPU usage is split between the Settings process and the MoUSO process. Every time I restore the non-active Windows Update window, 10 seconds of 50% CPU.
Rapidly clicking on the taskbar Settings gear icon results in a very slow minimize and restore of the Windows Update window. When I go to the Settings Home screen, it's not like this. Very very fast minimize and restore of the window. So even as of August 2021, MoUSO is broke and inefficient.
They need to fix what they already have instead of making a new OS that can only be installed on computers manufactured in 2016 or newer. But maybe that IS their fix for the broken MoUSO, buy new hardware.
EDIT: Oh, regarding your PS: above, check your Fast Startup in the power settings. I've seen numerous computer hang on the first, then a power cycle lets it boot properly. This happens after you select Shut Down and then boot up. A Restart shouldn't show the issue, only after a Shut Down. So disable Fast Startup if this applies to you.