If there are a high number of interviews starting that the counter value can vary greatly which makes it difficult to accurately load balance based on this.
There are two PercentProcessorTime values, System.PercentProcessorTime and Process.PercentProcessorTime. The former is calculated using the GetSystemTimes() Win32 call and the later is calculated using GetProcessTimes() Win32 call.
For both it keeps a buffer of the last five values and takes the average over the last 5 values that are within the last 30 seconds.
These counters should work quite well when PercentLoaded is called infrequently as the PercentProcessorTime is calculated over a long period. For a high number of starts per second across the whole cluster, which is what Ipsos will have, the PercentProcessorTime will likely be calculated over too short a period.