Hardware: I have a 2014 iMac 5K. It's got 24GB of RAM and a 2TB SATA SSD I installed myself last summer. Full specs here: https://support.apple.com/kb/SP707?locale=en_US
Software: Catalina 10.15.7 is installed. I have a total of three user accounts set up: one for my wife, and two for me (one for home, one for work). Her account is signed in under her Apple ID, my two accounts are both signed in under my Apple ID.
The Problem: When I first boot up the machine, it runs perfectly. Boots up very fast, accounts log in fast, everything runs quite smoothly. I use it for basic web stuff, email, iMessage, some Creative Cloud work, text-based work. All of this works very well, even more demanding stuff like Final Cut Pro and transcoding the occasional video.
But over a period of 3 or 4 days, even under very light use, things just get slower and slower. When I say "things" I mean specifically graphics on screen -- Mission Control and Fast User Switching become incredibly slow. Windows respond slower and slower to resize, lagging behind the cursor as if the machine is struggling to draw the graphics. When it gets bad enough, it can't even keep up with highlighting text in some apps and in web pages. If I log out of other accounts and pare it down to one, it seems to relieve the pressure a bit, but if I keep logging in and out of user accounts without restarting, it just slows down more and more over a few days until, again, it's basically unusable. Eventually I just have to restart -- at least once a week and sometimes more than that. Then it's very fast again until a few days later.
In all of this, Activity Monitor looks basically normal as far as I can tell. The only thing that stands out is WindowServer, which will spike up 70-100% and not really settle down much below 50%. But the fans never go crazy, the CPU load never looks bad, the memory pressure stays low (again, there's 24GB of RAM).
This issue has been with me for a couple of years now. I hoped that pulling out the Fusion Drive and replacing it with a pure SSD would help, but no such luck. I hoped Catalina would solve it, but not at all. What's so vexing about this is that this iMac runs SO well when I first restart it, but I can't keep it working that way unless I were to (I guess) run a single user account which I don't want to do. I have worked with separate home and work Mac user accounts for probably 8 years now, and never had issues with it, even on much more modest hardware.
I would also add this: I'm seeing a similar problem on my 2020 MacBook Air under Big Sur. Multiple users + staying logged in for a while = the machine slows down over the course of a week or so. This makes me wonder if there's just some inherent bugginess in the way MacOS handles multiple users these days.
Even if nobody has a solution to this, I'd love to hear about it if anyone else is seeing multiple user issues in MacOS these days.
edit: Per @Gilby's suggestion that this might be related to being logged into two user accounts with the same Apple ID, I've spent a week logged into only one of said accounts. Indeed, the severe slowdowns I was seeing have not manifested after a week of uptime working this way. So, I have to conclude that the issue is related to the "two user accounts, same Apple ID" setup.
It's a real shame, as I'd been working this way for some years without issue until I think Mojave or Catalina when it started to cause difficulties. I'm leaving this question up, for now, in case anyone has further insights or workarounds.