Running top shows that plasmashell was just eating CPU. The culprit, it seems, is a long-standing KDE bug that apparently rears its ugly head every now and then.
Temporary fix: run:
killall plasmashell; plasmashell &
... from a terminal, and it SHOULD drop back to normal. The problem, it seems, is with the notification icon(s) in your system tray ... get one, or have one with an animation (weather? Network? New mail?), and you'll get the spike.
Editing the /usr/share/plasma/plasmoids/org.kde.plasma.notifications/contents/ui/NotificationIcon.qml file seems to do the trick (your mileage may vary). Look for this section:
PlasmaComponents.BusyIndicator {
anchors.fill: parent
visible: jobs ? jobs.count > 0 : false
running: visible
}
..and change it to read:
PlasmaComponents.BusyIndicator {
anchors.fill: parent
visible: jobs ? jobs.count > 0 : false
running: false
}
Essentially, just replacing the "visible" with "false"