During testing of JQuery visual image effects on the home page of this website I noticed that images were loading after JQuery function calls had been made. This wasn't a problem on my PC with a speedy broadband connection but on friends computer the interface didn't look right.
Having used different load testing tools; I knew about the costly expense and time needed to set-up a good test environment. So I searched for something that would give me a basic web page testing tool and eventually stumbled upon Charles Web Debugging Proxy.
The Java based tool allows for testing and debugging of various applications such as Flash and Ajax. It also features a modem simulator and an accomanying firefox extension, which for me was a real boon as I could see each static file loading at various transfer rates.
This tool should be required in any web designers tool box and I recommed you give it a try.