50,000 users in 2019 hangs on boot but works on 2012R2

[InformB]

I've been using Windows server 2012R2 Standard for almost a decade now. I have apps running on the server which have created about 50,000 local users. These are local users created using WMIC scripts and eq. API's. The purpose is to allow for easy integration with IIS authentication. The app creates a local user account and IIS uses the built in Windows authentication to authenticate the user.

It's been running great on 2021R2 but I recently decided to upgrade to a 2019 Datacenter edition so I created a new 2019 server and used the same scripts to recreate the 50,000 users on the 2019 server.

However when I reboot the 2019 server it just hangs for hours at the spinning circle and takes up 100% CPU. It never get the login screen. I've never had this issues with 2012R2, to test it I spun up a new 2012R2 server and recreated the same 50,000 local users with the scripts and it took a few minutes to reboot but that was it.

Is there something different about 2019 server datacenter vs 2012r2 standard which causes the 2019 server to hang on boot when there are 50,000 local users? Is it trying to process all the users before presenting the login (I've set it to login automatically to an admin account but it never seems to get there). I'm at my wits end now and I'm open to suggestions on how to debug this further or what settings should I look at? I've checked out the group security and logon policies and they're both the same for the 2012 and 2019 servers. What am I overlooking?

They're both running on AWS with 1GB RAM. Thanks in advance and please don't hesitate to throw out whatever ideas you may have. I cannot change the architecture at this time and the easiest way to integrate IIS with authentication is to create a local user account. They don't need to have local login privileges only network login (for IIS), so these accounts are added to a custom Group on the windows server and removed from the standard Users group.

 

[InformB]

Looking at the procmon logs for the 2019 server more carefully I noticed that it's not that LSASS is taking a long time to parse the 50K users from SAM, it appears to be stuck in a loop!! LSASS keep reading all the SAM entries over and over again in a loop for 12 hours! Here's an excerpt from the logs showing when it ends one loop and then starts another loop. It's the exact same loop everytime. I can't see the end of the loop because procmon timed out after a few hours of logging but for those few hours it ends the last user, then reads the services NTDS and then restart the SAM user loop again. Anyone may any sense of this?

2:59:42.4202794 PM lsass.exe 680 RegEnumKey HKLM\SAM\SAM\Domains\Account\Users\Names SUCCESS Index: 39,993, Name: X
2:59:42.4202969 PM lsass.exe 680 RegOpenKey HKLM\SAM\SAM\Domains\Account\Users\Names\X SUCCESS Desired Access: Read
2:59:42.4203111 PM lsass.exe 680 RegQueryValue HKLM\SAM\SAM\Domains\Account\Users\Names\X\(Default) SUCCESS Type: <Unknown: 82518>
2:59:42.4203232 PM lsass.exe 680 RegCloseKey HKLM\SAM\SAM\Domains\Account\Users\Names\X SUCCESS
2:59:42.4203319 PM lsass.exe 680 RegEnumKey HKLM\SAM\SAM\Domains\Account\Users\Names NO MORE ENTRIES Index: 39,994, Length: 84
2:59:42.4203506 PM lsass.exe 680 RegCloseKey HKLM\SAM\SAM\Domains\Account\Users\Names SUCCESS
2:59:42.4203699 PM lsass.exe 680 RegEnumKey HKLM\SAM\SAM\Domains\Account\Users SUCCESS Index: 9, Name: XX
2:59:42.4203963 PM lsass.exe 680 RegQueryKey HKLM SUCCESS Query: HandleTags, HandleTags: 0x0
2:59:42.4204086 PM lsass.exe 680 RegOpenKey HKLM\SYSTEM\CurrentControlSet\Services\NTDS REPARSE Desired Access: Query Value
2:59:42.4204269 PM lsass.exe 680 RegOpenKey HKLM\System\CurrentControlSet\Services\NTDS SUCCESS Desired Access: Query Value
2:59:42.4204447 PM lsass.exe 680 RegQueryValue HKLM\System\CurrentControlSet\Services\NTDS\DirectoryServiceExtPt NAME NOT FOUND Length: 144
2:59:42.4204594 PM lsass.exe 680 RegCloseKey HKLM\System\CurrentControlSet\Services\NTDS SUCCESS
2:59:42.4204755 PM lsass.exe 680 RegOpenKey HKLM\SAM\SAM\DOMAINS\Account\Users\Names SUCCESS Desired Access: Read
2:59:42.4205331 PM lsass.exe 680 RegEnumKey HKLM\SAM\SAM\Domains\Account\Users\Names SUCCESS Index: 0, Name: XXX
2:59:42.4205462 PM lsass.exe 680 RegOpenKey HKLM\SAM\SAM\Domains\Account\Users\Names\XXX SUCCESS Desired Access: Read
2:59:42.4205604 PM lsass.exe 680 RegQueryValue HKLM\SAM\SAM\Domains\Account\Users\Names\XXX\(Default) SUCCESS Type: <Unknown: 42530>