Performance guru Steve Souders gave his keynote presentation, Cache is King! (slides), at the HTML5DevCon, besides being an extremely clear explanation of how caching works on the Internet and how to optimize your use of HTTP to get the best performance, Steve ran experiments that found some surprising results on what gave the best web site performance improvements.

In his base line test, page loads took 7.65 seconds (median of three runs). What change--Fast Network, No Javascript, or Primed Cache--would make the biggest performance improvement? It was Primed Cache.

• Fast Network - Using a fast FIOS network the load time was 4.13 seconds. Steve was surprised how big a difference this made, given how much work must happen in the browser. 
• No JavaScript - 4.74 seconds after disabling JavaScript. Both reduces transfers and skips parsing by the browser. Steve thought the effect would have been larger.
• Primed Cache - 3.46 seconds using a warm cache, less than half than the empty cache page view time because it reduced the number of HTTP requests and reduced the total transfer times. Key for mobile where higher latencies are common.

The implication being that caching is important so you must understand how HTTP caching works and how to make the best use of it. That's the rest of the talk.

Some key takeaways: