Boot time results with my 12700K (from when "Restarting..." message disappears to WinLogin screen):
12C/20T: 24 secs
After login, CPU utilization was under 10%. AMD process (long name) took 5 to 7% CPU.
1C/2T: 22 secs
After login, CPU utilization was 100% for about a minute or two. AMD process (long name) took about 56% CPU.
1C/1T: Took 3 to 4 minutes to get to login screen. After entering password, several minutes to get to desktop. When desktop appeared, the taskbar is blank. No start menu to click. Hard reset. Tried to go into BIOS too get back to full cores. Screen went black. Hard reset didn't work multiple times. Hard shutdown. This time it booted faster and logged in and Start menu button appeared. Right clicking the Start button took some time for the menu to appear. Clicked Task Manager at least two times. It never loaded in the 5 minutes of my patience threshold.
So yeah, have fun running your 13900KS with 1C/1T.
Hope a 7950X/7950X3D user will report their own Win11 experience with 1C/1T.