Using JMeter to test performance of Drupal with authenticated users
Tobby :
When building a site in Drupal (or any content management system, really), there are often concerns about how the site will perform. It's fairly easy to take measures to reduce the impact of anonymous traffic on server resources (cacheing, using CDNs, etc). However, much of what Drupal offers is the ability for users to interact with the site far beyond that of a traditional static site. So now you have a complete site built, and you're sure it can handle your anonymous traffic, and you want to test how well your site will perform when users log in and start doing things (such as leaving comments).

Alternative 

