So far in two weeks, am experiencing two separate cases of these latest MS updates being installed and causing extreme slowdown on computers. In the first case, it ended up being that Cryptographic Services were being engaged and using CPU excessively, and upon investigating it turned out this CS was being called by the Windows Update process. Killing that CS service, then stopping WU from trying to install the [older] KB5004945 (which I think is the predecessor to KB5004237) and then erasing contents of :\windows\SoftwareDistribution\Download (the WU working cache folder for downloaded updates) got the computer working.
Symptoms of that first incident were that the computer boots fine to the login screen, but as you enter the Windows login info, it then took 5 minutes or more just to get to the desktop, and from there, everything was extremely slow (5+ minutes to get Start menu, 5+ minutes to bring up Task Manager, etc., every.....single.....click....delayed reaction by several minutes.
In the second incident from last week (July 13 2021), all the same symptoms of extreme slowness, but no attribution to CryptoGraphic Services. In that case, we restored from a full system image backup, system works. Allowed this latest KB5004237 to install, and the problem began again. However, it didn't install properly I guess - got no errors though - it hung there at Installing: 73% for a LONG time, so we got impatient and rebooted to either cancel or restart the update. It "finished" however, with the pre-boot blue screen stuff saying "getting Windows ready" etc. As any update would, and later in Check for Updates, there was nothing new found. But the problem then came back within a few minutes.
FYI, booting to Safe Mode again worked fine, but of course WU doesn't run in Safe Mode so nothing useful to do there. Booted to the WinRE environment, did the Uninstall recent updates, picked the Quality update, and it removed successfully - and of course, no fix. No surprise there, when haave these tools ever done anything useful?? No restore points in System Restore found either.
We're now restoring from that image backup again, and will disable WU for a month or so.
Not sure why in my small world, with only two clients having this issue but where the issue is happening virtually identically on both their computers (computers are not at all the same models nor vendor), how is it that nobody else is reporting these problems. All I see are print problems, BSOD and such. I would think this problem would be common, or am I just that lucky? :)