user = % of controller CPU resources used by application processes
system = % of controller CPU resources used by system processes
idle = % of unused controller CPU resources
The following is taken from the Mobility Controller VRD. If you want to have a detailed analysis done, pursue your TAC case and see what they have to say specific to your version, etc.
The organization should consider the CPU to be highly utilized at 70% (begin monitoring regularly) and considered critical at 100% (being investigating) over a 5-minute period. Alternatively, consider using AirWave to track average CPU utilization over time. If the CPU utilization is sustained or regularly spikes above these thresholds, consider an upgrade to add capacity to the system