The Future of Web Performance at Google I/O: JavaScript

By Steve Souders, Member of Technical Staff

This post is one in a series that previews Google I/O, our biggest developer event in San Francisco, May 28-29. Over the next month, we'll be highlighting sessions and speakers to give Google Code Blog readers a better sense of what's in store for you at the event. - Ed.

In April I announced that I'm starting another book. The working title is High Performance Web Sites, Part 2. This book contains the next set of web performance best practices that goes beyond my first book and YSlow. Here are the rules I have so far:

1. Split the initial payload
2. Load scripts without blocking
3. Don't scatter scripts
4. Split dominant content domains
5. Make static content cookie-free
6. Reduce cookie weight
7. Minify CSS
8. Optimize images
9. Use iframes sparingly
10. To www or not to www

I'm most excited about the best practices for improving JavaScript performance (rules 1-3). Web sites that are serious about performance are making progress on the first set of rules, but there's still a lot of room for improving JavaScript performance. Across the ten top U.S. sites approximately 40% of the time to load the page is spent downloading and executing JavaScript, and only 26% of the JavaScript functionality downloaded is used before the onload event.

In my session at Google I/O I'll present the research behind rules 1-3, talk about how the ten top U.S. web sites perform, demonstrate Cuzillion, and give several takeaways that you can use to make your web site faster.