Apps' GUI becomes laggy/unresponsive, with some having flickering elements, when the desktop background is changed on Windows 10. Why?

I was super annoyed by the fact that regularly I got hangs/lag for any/all apps. Videos became slide-shows, apps responded with huge delay with the inputs buffered. All sorts of nastiness. Which got more severe over time. (Audio is never affected)
Some apps get flickering elements as well. I’m 99% sure they’re apps that use Micosoft’s GUI widgets. Total Commander, Notepad++, Foobar2000, even plain explorer.
After a long while I figured out it was because of the desktop background. I had it set to slideshow and so it was periodically changed. But it happens just the same if I set a background manually.

After I set a non-changing image as background, the issue stopped happening.

I don’t have the slightest idea what might be causing this wacky issue.

Also the animated lockscreen is bugged. (May be related) When I lock the computer most times I get the login screen’s background statically, with no text whatsoever. When I press something to log in, for a split second the normal animated lock
screen appears, which apparently runs in the background under this glitch screen.

I also had BSODs, but it seemed to have come in a wave, because I didn’t have any for a while.