Debugging a Slow Web App

I got an email today from one of my clients, letting me know that one of his web apps was down. He said he was getting an error and asked me to take a look.

In the end, I was able to fix it and get it running faster than ever. What caused it turned out to be a huge surprise to me. I thought I'd outline the steps I went through here, to try to help others trying to solve similar problems in the future.