I had high CPU usage while "Dolphin" window was opened. Later I figured out that problem was with "size of contents" option which I manually turned on. And specifically with symlink "z:" which point to "/" in "$HOME/.wine/dosdevices/" folder. That symlink Wine creates by default during configuration.
STEPS TO REPRODUCE
1. Settings > Configure Dolphin > View Modes > Details. For "Folder size displays" select "Size of contents, up to 4 levels deep"
2. F4 to open terminal
3. Create symlink to "/proc" with command and "ln -s /proc test_size"
Dolphin start eating up to 48% of CPU and at some point will show some strange value like "21,6 PiB"
Probably Dolphin shouldn't inspect symlinks and /proc directory
Linux/KDE Plasma: KDE neon User Edition
KDE Plasma Version: 5.21
KDE Frameworks Version: 5.79
Qt Version: 5.15.2
FYI the folder size counting takes into account folder hiding, i.e with hidden folders hidden you wouldn't experience this issue.
Activating option "Size of contents, up to 10 levels deep", leads to 100% cpu usage in a seemingly infinite loop (can last hours). Some dirs also report a much overestimated size.
On one example, a dir (15 levels, 1045 dir, 41233 files, `du` size of 169 GiB) is reported as 512.2 TiB.
Selecting a lower number of levels (2-3) does not trigger the problem (size is underestimated, as expected).
The tree might contain circular symbolic links. fs is brtfs. I have not tried browsing /proc and did not create symbolic links there.