Energy saving may throttle down the CPU frequency, which Renoises CPU meter does not take into account. This is very confusing, so we probably should try to fix that at some point. What the meter in Renoise displays is the “real, current” load of the CPU and not the possible one when it’s running at full speed, which is what the OS process viewer shows.
See for example From Os X To Ubuntu on how to force the CPU to run at full speed.
For audio production, throttled down CPUs can be a bad thing, cause it may take to long to speed up the CPU frequency again when you need it. When for example a heavy DSP kicks in at a certain point in the song…