Starting in vCM 5.7, you can use the "Default maximum number of concurrent agent activities per host" setting in Administration > Settings > General Settings > Collector in conjunction with vCenter collections. The agent collection process is not typically very cpu intensive, but there are some collections (like Event Viewer) that can cause spikes. Since this setting is a new feature with 5.7, there is no established baseline for the value. If you have load balancing in vCenter, there may be some better consistency the effect of this setting. I would think that the value could start at 25% of the number of guests, but please experiment to find the optimum value. Keep in mind that too low of a value can adversely affect how long collections will run.