Running Plasma System Monitor (plasma6-systemmonitor) causes excessive CPU load on X11 (KDE 6)

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Running Plasma System Monitor (plasma6-systemmonitor) causes excessive CPU load on X11 (KDE 6), but it works OK on Wayland.
Tested with Plasma 6 and IceWM on Leap 16.0.

I expected right the opposite behaviour.

KDE page:

Package:
https://download.opensuse.org/distribution/leap/16.0/repo/oss/x86_64/plasma6-systemmonitor-6.4.2-bp160.1.2.x86_64.rpm

Bug report: 1258794