This issue is very likely not related to plasma-systemmonitor directly but it is the only tool from the KDE suite I have available.

For a program that is supposed to query statuses of system utilization once it a while, the program seems to utilize the CPU a lot. On an otherwise completely idle system, plasma-systemmonitor will use anywhere between approx. 15 - 25 % CPU when showing the list of all active processes. I don't have the CPU usage readout normalized so the maximum would be 800 % on my Intel i7-3632QM.

For comparison, similarly configured htop never uses more than ~8 % CPU. For further comparison, on this system, Firefox playing Full HD video stream needs about 30 % of CPU time (all non-normalized).

According to the profiler, most of the time is spent inside the Qt signalling machinery. I built both plasma-systemmonitor and libksysguard with debugging information and frame pointers enabled so the perf log is hopefully accurate enough.

I installed plasma-systemmonitor only when I started testing Plasma 6. I don't recall any of the Plasma 5 resource monitoring tools to ever even show up on a CPU-usage-sorted list and I was using those tools a lot.

I attached some screenshots to illustrate the problem better. Mind that the sampling rate and CPU frequency scaling obscures the readout a bit.