Testing and proving that site slowdowns are not due to server issues

I have a rather vocal web hosting client that has emailed me several times saying that his sites are loading slowly. When I check everything seems fine, loads fast, and none of the other clients are seeing any problems. I run tests with online tools like the Pingdom tools and multi location testers like http://cloudmonitor.ca.com/en/checkit.php and everything says they are fine and running fast.
I have two questions related to this.
First, just in case it is an intermittent thing that…
Testing and proving that site slowdowns are not due to server issues

See original post by Maestroc