Monitoring your load generator client, when the server CPU% is under utilized

Recently, I am using one Virtual Box(VMware/2 Cores / 8GB) to conduct performance/load testing. With more and more load Jmeter generate, It seems the server's CPU% is not able to go up...
However, I noticed the my vitual-box client's CPU% has reached to 90%+. After changing to a physical and more powerful machine, everything is fine, server side CPU% can go up with increasing payload. I am not often to monitoring load generator client during my testing previously, and they are usually works fine as its physical machine.
You have to monitor your load generator machine as well when you find the server CPU% is under utilized(stay the same with even higher user load) and Server response time keep increasing, especially if you are using a Virtual Box to do performance testing, then please be caution!!