In the early days of virtualization, the core focus of virtualization was primarily consolidation. You could achieve quite high consolidation ratios, with some even as great as 20 to 1. This consolidation worked great for applications like file and print servers, development workloads, or other very lightly used servers. The virtualized servers that hold these servers are technically overcommitted on resources, but the workloads are so low that the end users would not notice the effects.
However, as more and more business-critical workloads are virtualized, maintaining this same level of consolidation is guaranteed to rear its ugly head in the form of performance degradation of the virtual machines. Most VMware administrators realize that resource overcommitment is slowing down their most intensive servers.
How can this be, you ask?
Few administrators know about a simple metric called CPU Ready.