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From time to time and perceived randomly the plasmashell process uses all the remaining CPU capacity for a few seconds pushing it to 100 % overall. It seems to be related to using the browser (firefox-developer-edition), but I'm not sure. Unfortunately I have no idea what's causing this, but I suspect it's a bug, because I observed this on my desktop computer as well as on my laptop, both running Arch Linux on testing and KDE unstable repositories. Both using intel-ucode and kept up-to-date daily.
Desktop:
Intel Core i5-3570
5.12.4-zen1-2-zen
plasma-desktop 5.21.90-2
Laptop:
Intel Core i5-4200U
5.12.4-zen1-2-zen
plasma-desktop 5.21.90-2
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