Windows Booting very slow after 2004 update ,

Windows isn't becoming slower. That's a misconception !

I'm a programmer and I had (and still have) Windows 10 from it's first release. I've gone through all the updates and versions and the system kept it's parameters without exceptions.

Now I've installed v 2004, It's a fresh install because lately I've done some experiments with my PC and .... you get the picture.

The system has the same speed, with a slightly noticeable "pause" when you start a program or app than the 1909.

If you're still running on HDD, please change it immediately with an SSD.

Two things I've always done, right after a fresh install or major update of Windows:

1. Stopped and disabled SysMain service, or SuperFetch how it was called in early versions.
2. Stopped and disabled indexing service.

As a professional, I've never understood the role of SuperFetch (SysMain), other than keeping my drive bussy all the time. And the indexing service is just lame. I mean, if I don't know to organize and label my stuff it's my problem. That's my point of view, anyway.

Right after stopping SysMain, you drive will breath again. Trust me on this one.

Well of course you don't have the same experience, you are working off of a clean installation.

The problem is for those who ran the Windows update that upgraded their systems to the current 2004 Build.

It indeed slows the system down, my DELL Inspiron 5000 laptop on a 2 core 4 thread Intel 4th gen i3 ran pretty snappy since I bought it in 2016, and have gone through several updates.  But this update has the system performing like I just swapped out my system for previous DELL XPS M1210 from 2006 that used a Core 2 Duo. 

Yes my system is on a HDD but that has never been the problem for this laptop.

Suggesting upgrade to SSD won't address the problem, that only adds to people's issues. 

For starts a lot of laptops (mine included) are not upgradable in that way, I can't swap in a SSD, the laptop is completely closed off. 

2nd users who can are looking at a complete re-installation and will have a lot of work on their hands as far as backing up files in prep for transfer.

3rd.  It's the build.

I just went to Windows Update, click on Update History > Uninstall Updates and used the option to Restore my PC to previous version of Windows and proceeded.

I am typing this now back on Build 1903 and it is lightening fast.    

I suggest others do the same if you haven't deleted your Windows Update files (from Disk Cleanup) you might still have the option under Start > Settings > Windows Update , to do what I did above and go back to the previous version of Windows.

I agree that there are services using the CPU that end up sneaking into peoples computer via some installer with a bunch of strings attached. There are also some windows services that should be stopped if they don't effect the user., especially if it enhances performance. It drives my crazy when I see some Adobe thing or "helper" or some other junk sitting there waiting to see if the user might need it and more often than not they don't. I've also noticed that things like the EA Origin game launcher simply stays on in the background after the user quits. I assume it's used to spy on the end user as I can see no reason for many of the programs that hang out in the background being useless.

I don't agree though with your statement :"Windows isn't becoming slower. That's a misconception !" . I wouldn't tell people that their computers aren't slowing down after certain updates or application installs.

It must be nice to have the particular set of coveted hardware that doesn't slow down. You probably stumbled upon the right combo used by Microsoft for their programming and tests. I haven't been so fortunate. I now have to go through and examine all my services and updates to figure out why my computer hit a wall in the last few months. You could tell me that my concept of slowness associated with Windows is wrong if you want but when my computer goes from booting up quickly and behaving as one would expect a computer to behave, to drawing icons one at a time and displaying a folder at least 20 seconds after I click it, I'm going to say it's slow. I have an SSD and I had all my apps installed and working for quite some time. Something happened.

I just want to say jumping to the conclusion that everyone else's hardware must work like your hardware isn't going to be helpful to people.

I had updated to Win 10 version 2004, (build 19041.450), an everything had slowed to a crawl -- loading sites, running apps, working on a normal excel or word spreadsheet.  I had tried a couple of the suggestions, finally recovered to version 1903 (build 18362.1082) and all returned to normal.  Even an Excel problem of freezing on Cntrl-C was suddenly solved.  Thank you!

This did nothing to help with slow boot time. This is probably something that will never get fixed.

For over a month I tried many recommended fixes without success - including enlisting the help of Geek Squad. Nothing worked.  Finally decided to reset my computer opting for the process to delete all files, download and reinstall Windows 10. My computer now runs like a champ, fast as though new - albeit it took me a day to reinstall flies and programs (apps) that I had backed up on an external drive.  Saved me from having to purchase a new laptop - at least for now.

This did nothing to help with slow boot time. This is probably something that will never get fixed.