That's not unexpected at all.  Depending on the processor type and model, decreasing CPU frequency when processor is idle may or may not provide any power savings. Many recent Intel processors support C1E state, which automatically reduces core frequency when the whole package is idle (all cores are idle).  This means that reducing frequency using DVFS algorithm in ESX when package is relatively idle is not going to help since the hardware is already doing it.  However, even on those systems, you can still expect to see some power savings when there is some load.  C1E only works in cases when packages are almost completely idle, and DVFS algorithm in ESX can help to save power when load is somewhere below 60% (which is a configurable threshold by the way).  DVFS will continously monitor CPU utilization and make sure that core frequency is not unnecessarily higher than what is currently required.  So, for example, if CPU utilization is 25% when CPU is running with its highest frequency, ESX can reduce frequence almost in half thus potentially increasing CPU utilization to 50% but still leaving enough headroom for sudden changes in CPU load.  DVFS algorithm used in ESX is very conservative, and it aims to save power while not affecting performance.  Hope this helps.