I assume you're looking at CPU utilization, but that isn't always the case. Sometimes the CPU will be spending a lot of time waiting on IO. For instance, if the CPU is waiting on information over the network it won't matter how much CPU power you have on tap. Also, it could be a small subsystem of your CPU that's overloaded while the "CPU Utilization" is being reported for the entire processor, which, on the average could be low...