In the old KSysGuard, you could override the 1 second update interval via View → Tab Propierties. I’ve chosen 0,5 sec as it is drawing more precise graphs. I couldn’t find this setting in the new System Monitor.
This affects me very strongly as well as on my 32 cpu system, the system monitor program uses about 50-60% of one core on the default overview tab (as measured with htop, which only uses 2-4% for comparison).