In the Pluralsight Silverlight 3 Fundamentals video course, the instructor suggests a tip to test a custom progress bar by mimicking a slow web server on the local computer. This can be done by generating modem speeds with Fiddler, a HTTP proxy, using the menu option Rules > Performance > Simulate Modem speeds. The speed can also be customized. This technique is more convenient than using the System.Threading.Thread.Sleep method to create an artifical delay with code.
Firefox Throttle plug-in also allows you to cap download/upload rates and monitor current bandwidth utilization. This plugin seems to be currently removed from the Mozilla add-ons directory but as per this Firefox support Forum its available on the Mozilla FTP server(direct link to .XPI).
Also see:
FiddlerCap makes HTTP Traffic Capture easier