Comment by nonethewiser

> To test DOM access speed simply compare processing speed during test automation of a large single page app with all DOM references cached to variables versus the same application with no such caching. I have done this and there is a performance difference, but that performance difference cannot be noticed until other areas of the application are very well optimized for performance.

This was my instinct. Remember, the author is a hammer. His role is to find performance issues in the frontend. His role is not to find performance bottlenecks in the whole system.