On Matt Kalinowski's blog, there's a quick post with some code to help you test just how fast your user's browser speed is as they hit your site.
So, you have a site that offers both an enhanced and standard style page. However, you realize that not every web user is going to know if they would be best suited for either version. Therefor, this speed test will allow you to determine their speed. This could also be used in other situations as well, but the one stated above is the one I found most appropriate.
Basically, the script works with the microtime functionality in PHP to do some speed tests for the amount of data given divided by how long it takes that data to get out to the user.