Charles has basic load testing capability. In order to load test first start a new session in Charles and record a browser session navigating around the website. You can fill in forms and perform any other tasks on the website, as long as they are reproducible. After making this recording I suggest saving the session so that you can come back to it.
Choose the site node from the tree (or any other node); right-click and choose Repeat Advanced. You can then specify some of the characteristics of your load testing including how many times to repeat each request and with what concurrency level. The concurrency level represents how many users are visiting the site, and the iterations how many times. Choose to show the results in a new session.
A new session will be opened in Charles which will show only the requests from your load test. You can then click on various nodes (including the site node) and use the Overview, Summary and Chart views to look at your results.