When you enter a website name in your browser or click on a google result, you start a cascade of events to unfold. Your request various pieces and components that make up the webpage from a remote server which hosts that website. Those pieces are sent back to you, and your browser assembles them.
There are many moving parts in that process. Anywhere along the way you can hit a snag, slowing down the overall process of that page displaying. Website Optimization attempts to identify all of those processes and components, and organize them by slowest to fastest. This allows us to focus our attention on the slowest part of the process. Like a physician looking at your vascular system, it allows the performance expert to identify and then fix those pipes that are slowing you down.
Since website performance has been shown to directly influence customer retention, conversion, and user experience, overall website performance and optimization are key to your business success.
Sean Hull asks on Quora: What is website optimization and why is it important?