Bringing a Virtual Host to its Knees

I recently got called to examine some performance issues that were happening to a VMware VDI Cluster. I was told all the hosts in the cluster would run at 100% CPU utilization for an extended period of time and the client would like an explanation and recommendation. I pretty much had a good idea what the problem was before I ever started looking at hosts. I know this topic has been covered many times before but it does not seem like it has been covered enough.