Group calc is a law unto itself and whilst I’d love to explain it, that’s for another time. However normal group calcuations occur (on a default installation of SCOM) every 30 seconds. If you work in a corporate environment with a lot of SCOM groups then I’d bet that you’d have changed this value.
So if you want to change this value of 30 seconds to something a little more reasonable then load up “Regedit” and look for the following DWORD Value:
HKLMSOFTWAREMicrosftMicrosoft Operations Manager3.0GroupCalcPollingIntervalMilliseconds
As the name suggests the value is in Milliseconds. I have mine set to 900,000 or 15 Minutes.