Windows Booting very slow after 2004 update ,

I tried disabling SysMain (aka SuperFetch/Prefetch) but this did not help. I re-enabled it and over the next few days the boot time gradually came down to what it was before updating to version 2004.  I am guessing but what may have happened in that initially everything was loaded on start-up, but then gradually SysMain or something else only initially loaded those Windows programs which I use. 

Incidentally the version 2004 major update was followed a few days later by the 20H2 minor update. I understand that subsequent updates will use the yyH1or 2 format.

@DanDScan I have copied your useful post informaton about the registry change, to the Microsoft "AskPerf" blog post, set up by the performance group manager. Hopefully they might review your findings, but I am not sure what "exposure" it actually gets.  I hope you don't mind me doing that, but I suspect that the "Performance group" is driven from above, rather than user input, but is worth a try.

It is here if you want to take a peek:

https://techcommunity.microsoft.com/t5/ask-the-performance-team/welcome-back-askperf/bc-p/1920044

It might also be worthwhile you putting your useful info into the Feedback Hub, in the hope that the more it is mentioned, and "upvoted", the more MS might take an interest.

I did notice something similar. In mid-July when I went from v1909 to v2004 the boot time became a diabolical 10 minutes but over a period (not sure how long) it dropped to an atrocious 6 minutes where it stayed. Only in mid-September did I try to make it better, like disabling SysMain (since re-enabled), stopping only at the more extreme solutions like installing an SSD, re-installing Windows 10, or going back to v1909, which each seemed too much effort for a potential saving of 5 minutes per boot.

No problem at all in forwarding my post.

I am sure you will have done it, but just in case you haven't - run Disk Cleanup (including Clean up system files). I found and deleted 26GB of files after the upgrade to version 2004.