On Debian 11 stable with kernel 5.10.0-11-amd64 + KDE Plasma 5 all up to date when i start ksysguard 5.20.5-2 (up to date bullseye/main repo) then it start 3 processes and 2 use 100% of the CPU thread. This happens only on Process table tab when there is active either Upload or Download column. <<< Someone mention that developers known about this issue when they developed the feature. I see the issue for example during 6MB/s traffic and when dropped to 2MB/s CPU usage is unchanged). Happens even ksysguard is on background. I think this is not good. When you search "ksgrd_network_helper CPU" you may see people for multiple distributions have the issue. This feature is a fail, because in my case it displays network usage for like 3 apps, total like 200kB/s while my real usage is like 6MB/s used by much more apps.. + it is more-less wasting 2 CPU threads.
ksysguard is no longer maintained, in Plasma 6 there is the Plasma system monitor for this task. If your issue still happens with the Plasma 6 replacement, please re-open and we can move this bug to the new product, thanks!