My Installed Updates screen informs me that the latest (and only) "Update for Microsoft Windows" corresponds to KB4100403. Reading this thread, it's apparent there are more up-to-date versions since my version was released,...
I don't know if you have in mind my mention of "KB4100403 and then KB4287903" or something else, but KB4100403 is the only update for Windows I have too. KB4287903 was a security update for Adobe Flash Player, but still seemed to cause problems on my machine. At least, things did improve for a while after I uninstalled it.
I did eventually figure out that the supposed "solution" (KB4103721), mentioned in the OP, probably was included in the version of 1803 which I received on May 16.
On another subject:
I'm no expert in the field, but since I've noticed my system grinding to a shuddering halt as the day wears on, I'm tending to suspect it might be caused by a memory leak.
There's supposed to be a seemingly easy way to check, but I can't yet see how to set up Performance Monitor according to instructions.
https://docs.microsoft.com/en-us/windows-hardware/drivers/debugger/determining-whether-a-leak-exists