Start by checking what windowing system you’re using as its a fundamental part of problem solving. It’s a little confusing how to do this, the top answer in this Stack exchange thread works well.

If you’re running the latest KDE then you’ve almost certainly been moved to Wayland and that will be the source of your problems. Wayland and Nvidia drivers don’t work well together, and KDE have defaulted to Wayland in the latest release. I have had very similar issues to you with the move to wayland and have not been able to fix them - they’re too fundamental and depend on updates to wayland and/or Nvidia drivers.

I know you don’t want a solution but there isn’t one at the moment, so you’d be wasting your time. The solution is to log out, then on the log in screen select Plasma (X11) as your session and log in again.

Personally I have had to abandon KDE as I get a different set of problems in X11. I’m on OpenSuSE Tumbleweed so have little choice inrolling back to the previously functioning version of KDE - I’m using Cinnamon instead and contemplating switching to a different Linux distro, probably OpenSuSE Leap in favour of stability over cutting edge.

Meanwhile I have the latest KDE running on another device with AMD GPU without issue.

In terms of when it’ll be fixed, there is a change being made to Wayland which will effect how it and the Nvidia drivers interact (something called Explicit sync). It’s just been merged into wayland so presumably will appear downstream in the coming next few months in rolling distributions. There have been articles suggesting this is going to fix most problems but personally I think this is a little brave but fingers crossed.

Try switching to different versions of your graphics driver and/or kernel. Nvidia cards get really finicky about the version matchups, especially as they age. Try different combinations of the versions that are available via pacman, and maybe it’ll work. You may need to start keeping an eye on updates to your kernel and graphics driver to see if a new update fixes your issue. Welcome to life with an nvidia card. I bought an nvidia card once in 2013. By 2016 I had to start playing this game on upgrades. At one point, the graphics driver was causing kernel panics until I downgraded both and waited a few months. Very happy with AMD.

Look in /var/log/Xorg.0.log for Xorg errors.

Check if OpenGL is okay by running glxinfo (from the package mesa-utils) and checking in the first few lines for “direct rendering: Yes”.

Check if Vulkan is okay by running vulkaninfo (from the package vulkan-tools) and seeing… if it throws errors at you, I guess. There are probably some specific things you could look for but I’m not familiar enough with Vulkan yet.

You could sudo dmesg and read through looking for problems, but there might be a lot of noise to sift through. I’d start by piping it through grep -i nvidia to look for driver-specific stuff.

Might be worth running nvidia-settings and poking around to see if anything seems amiss. Not sure what you’d actually be looking for, but yeah.

Sometimes switching from linux and nvidia to linux-lts and nvidia-lts can help if the problem is in the kernel or driver. Remember to switch both of these at the same time, since drivers need to match the kernel.

You could also try switching from the nvidia drivers to nouveau. Might offer temporary relief and help narrow down where the problem is, at the expense of probably worse performance in heavy games. Ought to be fine for 2D gaming and general desktopping.

Trying a different window manager is always an option. Don’t know how much hassle that is when you use a full DE; I’ve always been the “just grab individual lightweight pieces and slap 'em together” sort so I don’t have any real experience with KDE. But yeah. Find out what the right way to change WM is for your system, then try swapping over to Openbox or something minimal like that and see what happens.

Related to WM/DE, it could be an issue with the compositor maybe. Look up whatever KDE’s compositor is and see if you can turn it off and run a different one?