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?