Created attachment 156716 [details]
Video of the described problem

Check the size of the blue bar in the panel, depending on whether there are any windows drawn or not. Any time I restore the konsole window, you can see what two processes were peaking a second before.

Ignore the numbers under the bar, those are network speeds.

Sorry for the phone video, I couldn't get a proper screen recording in Wayland.

In my first comment it says "Graphics Platform: X11", but that's because I reported the bug from X11. This is obviously a Wayland specific bug.

I don't se any abnormal CPU usage from plasmashell in that video; only kwin_wayland. Is this the case, or am I missing it?

The video is quite bad because I had to compress it into less than 4 MB, but if you look closely at the moments when I click on konsole (for example, 29 or 41 seconds into the video), you can see that plasmashell was higher than 80%. It's the first row, in blue. It drops very quickly to low levels, because it only peaks when all windows are minimized (or closed).

Created attachment 156876 [details]
htop in a virtual terminal

Since opening any window (like Konsole) makes the peaking processes go to normal levels almost instantly, I'm attaching a picture of htop in a virtual terminal, where I can observe the bug without "disturbing" it.

BTW, going back to Plasma (as in Ctrl+Alt+F7) gets me a crashed desktop, where everything is black, except for the mouse pointer. No panel or icons, no KRunner, nothing. I can only kill the X session, and still have to reboot because most user processes (plasmashell and kwin_wayland among them) stay alive anyway.

Interesting, thanks a lot for the extra info. I wonder if this is related to Bug 466551.

I got it wrong: it's not whether windows are open or not, it's whether they are focused or not. So if I have any window selected, the CPU remains at normal levels, but as soon as I click on the desktop background, thus deselecting the window, plasmashell and kwin_wayland go crazy.

More interestingly, I don't have this bug in other users of the same computer. So it's definitely something in my particular user that is triggering this.

I'm willing to try to narrow it down by resetting my .local or .config files until it disappears, but I don't want to mess with my home user too much, as this is my work computer. Any ideas as to what files I should try resetting first, given that plasmashell and kwin_wayland seem to be the two processes involved?

I have the same issue with wayland. And similarly, another user doesn't. I noticed it thanks to power monitor: https://github.com/atul-g/plasma-power-monitor. With all windows minimized, the power usage stays at 12-17W vs regular 5-7W. 
Not sure which of my configs would be responsible. I tried removing some widgets but no luck. 
I've got laptop with AMD Ryzen 5 5500U.

Can you attach your .config/plasma-org.kde.plasma.desktop-appletsrc  

There are some wallpapers that try to animate if no windows are shown, that sort of thing would match the symptoms.

Created attachment 159392 [details]
plasma-org.kde.plasma.desktop-appletsrc file

Thank you for looking into it, David. Here's the file.

In any case, I later figured out that the bug doesn't only take place when there are no windows shown, it happens when no window is focused. That is, if I've got a window open but I click on the desktop (thus unfocusing the window), the plasmashell and kwin_wayland processes go crazy too.

Created attachment 159395 [details]
plasma-org.kde.plasma.desktop-appletsrc

I attached my plasma-org.kde.plasma.desktop-appletsrc  file too as my symptoms are exactly the same. It's when no windows is in focus that I get the issue. I tried removing all applets but it didn't help.

Dear Bug Submitter,

This bug has been in NEEDSINFO status with no change for at least
15 days. Please provide the requested information as soon as
possible and set the bug status as REPORTED. Due to regular bug
tracker maintenance, if the bug is still in NEEDSINFO status with
no change in 30 days the bug will be closed as RESOLVED > WORKSFORME
due to lack of needed information.

For more information about our bug triaging procedures please read the
wiki located here:
https://community.kde.org/Guidelines_and_HOWTOs/Bug_triaging

If you have already provided the requested information, please
mark the bug as REPORTED so that the KDE team knows that the bug is
ready to be confirmed.

Thank you for helping us make KDE software even better for everyone!

Marking it as REPORTED again.

The bug is still present in 5.27.8 for me. I tried removing plasmashellrc, kwinrc, kwinrulesrc, and defaulting on most settings in systemsettings but it made no difference.

Created attachment 161801 [details]
Htop in unfocused konsole

I removed all the possible plasma configs, I started setting up everything to my preference and I think I know what causes the issue. Could someone test too?

To replicate:
go to clipboard settings and tun on 'Keep the selection and clipboard the same' and 'Text selection: Always save in history'

I can confirm that your method to replicate the bug is working for me. Thank you for that, I wouldn't have guessed it was the clipboard settings in a million years!

For anyone willing to look into it, a couple of additional comments:

1) It's specifically the 'Text selection: Always save in history' option that triggers the bug. That is: 'Keep the selection and clipboard the same' together with 'Text selection: Only when explicitly copied' is fine.

2) To experience the bug, you need to select 'Text selection: Always save in history', and relogin. Once you do that, your plasmashell and kwin_wayland processes will peak when no windows have the focus. However, in order to stop the bug, just select again 'Text selection: Always save in history', and the processes will go back to normal immediately. No need to relogin in this case.

(In reply to David de Cos from comment #18)
> I can confirm that your method to replicate the bug is working for me. Thank
> you for that, I wouldn't have guessed it was the clipboard settings in a
> million years!

It is bonkers. I've had some unrelated issue that made me reset everything, otherwise I wouldn't think to check out the clipboard config in a million years. 
Hopefully it will now get fixed as clipboard history is a neat feature to have.

Interesting, I cannot reproduce the issue in Plasma 6 Wayland with that exact combination of settings.

(In reply to Nate Graham from comment #20)
> Interesting, I cannot reproduce the issue in Plasma 6 Wayland with that
> exact combination of settings.

It's good to know it's not being carried to Plasma 6!

As for Plasma 5, I think I can say it's always reproducible, at least with Plasma 5.27. I have just reproduced it in two computers, with different:
- OS (Opensuse Tumbleweed and Kubuntu 23.04)
- Users (my primary user and a newly created one)
- Plasma versions (5.27.9 and 5.27.4)
- QT versions (5.15.11 and 5.15.8)

Cool. At the moment almost 100% of the Plasma dev team's focus is on Plasma 6, so unfortunately I doubt anyone will be able to investigate and produce a fix for 5.27.10 (which will be the last Plasma 5 bugfix release). Let's call it fixed for Plasma 6! At least there's that. :)

The bug is present in 5.92.0

(In reply to kerown from comment #23)
> The bug is present in 5.92.0

That's unfortunate... I was really eager to switch to Wayland as soon as Plasma 6 landed, but that's a showstopper for me. However, I was under the impression that Nate checked it and couldn't reproduce the bug... maybe it reappeared in a more recent version of Plasma 6?

Same with RC2. As soon as there is something in the clipboard (any text selection in that case) the CPU spikes with windows minimized. I'm guessing it won't be high on priority list with other more critical bugs needing addressing before Plasma 6 release.

I can confirm this bug is still present in Plasma 6.0.1, in my case in OpenSUSE Tumbleweed.

(In reply to David de Cos from comment #26)
> I can confirm this bug is still present in Plasma 6.0.1, in my case in
> OpenSUSE Tumbleweed.

Same with 6.0.2

This is still present on Plasma 6.0.4, Frameworks 6.1.0, Qt 6.7.0.

As I said in a previous comment, I can reproduce this with different users, distros, computers, etc, so I bet this is general. Just need to go to Clipboard settings, select "Text selection: Always save in history", and relogin.

I'm attaching a new video I just recorded with Spectacle, in case I can lure some hero developer into having a look at this ;-)

Created attachment 168841 [details]
Video showing the bug in action

See how the processes differ between this two situations that alternate in the video:
- Click on konsole (a window is focused, normal cpu usage)
- Click on the desktop background (no windows focused, high cpu usage)