Even Faster Web Sites best practices for faster pages

1. Even Faster Web Sitesbest practices for faster pages Steve Souders souders@google.com http://stevesouders.com/ Disclaimer: This content does not necessarily reflect the opinions of my employer.

2. The Importance of Frontend Performance 9% 91% 17% 83% iGoogle, primed cache iGoogle, empty cache

3. Time Spent on the Frontend

4. The Performance Golden Rule 80-90% of the end-user response time is spent on the frontend. Start there. greater potential for improvement simpler proven to work

5. 14 Rules • Make fewer HTTP requests • Use a CDN • Add an Expires header • Gzip components • Put stylesheets at the top • Put scripts at the bottom • Avoid CSS expressions • Make JS and CSS external • Reduce DNS lookups • Minify JS • Avoid redirects • Remove duplicate scripts • Configure ETags • Make AJAX cacheable

6. YSlow High Performance Web Sites

7. http://conferences.oreilly.com/velocity/20% discount: vel08st

8. High Performance Web Sites, Vol 2 • Split the initial payload • Load scripts without blocking • Don't scatter inline scripts • Split dominant domains • Make static content cookie-free • Reduce cookie weight • Minify CSS • Optimize images • Use iframes sparingly • To www or not to www

9. Why focus on JavaScript? Yahoo! Wikipedia eBay AOL MySpace YouTube Facebook

10. Scripts Block <script src="A.js"> blocks parallel downloads and rendering http://stevesouders.com/cuzillion/?ex=10008 What's "Cuzillion"?

11. Cuzillion'cuz there are a zillion pages to check a tool for quickly constructing web pages to see how components interact Open Source http://stevesouders.com/cuzillion/

12. Initial Payload and Execution 26% avg 252K avg

13. Split the initial payload split your JavaScript between what's needed to render the page and everything else load "everything else" after the page is rendered separate manually (Firebug); tools needed to automate this (Doloto from Microsoft) load scripts without blocking – how?

14. MSN.com: Parallel Scripts MSN Scripts and other resources downloaded in parallel! How? var p=g.getElementsByTagName("HEAD")[0]; var c=g.createElement("script"); c.type="text/javascript"; c.onreadystatechange=n; c.onerror=c.onload=k; c.src=e; p.appendChild(c)

15. Advanced Script Loading XHR Eval XHR Injection Script in Iframe Script DOM Element Script Defer document.write Script Tag

16. XHR Eval varxhrObj = getXHRObject(); xhrObj.onreadystatechange = function() { if ( xhrObj.readyState != 4 ) return; eval(xhrObj.responseText); }; xhrObj.open('GET', 'A.js', true); xhrObj.send(''); script must have same domain as main page must refactor script http://stevesouders.com/cuzillion/?ex=10009

17. XHR Injection varxhrObj = getXHRObject(); xhrObj.onreadystatechange = function() { if ( xhrObj.readyState != 4 ) return; var se=document.createElement('script'); document.getElementsByTagName('head') [0].appendChild(se); se.text = xhrObj.responseText; }; xhrObj.open('GET', 'A.js', true); xhrObj.send(''); script must have same domain as main page http://stevesouders.com/cuzillion/?ex=10015

18. Script in Iframe <iframe src='A.html' width=0 height=0 frameborder=0 id=frame1></iframe> • iframe must have same domain as main page • must refactor script: • // access iframe from main page • window.frames[0].createNewDiv(); • // access main page from iframe • parent.document.createElement('div'); http://stevesouders.com/cuzillion/?ex=10012

19. Script DOM Element var se = document.createElement('script'); se.src = 'http://anydomain.com/A.js'; document.getElementsByTagName('head') [0].appendChild(se); script and main page domains can differ no need to refactor JavaScript http://stevesouders.com/cuzillion/?ex=10010

20. Script Defer <script defersrc='A.js'></script> only supported in IE script and main page domains can differ no need to refactor JavaScript http://stevesouders.com/cuzillion/?ex=10013

21. document.write Script Tag document.write("<scri" + "ipt type='text/javascript' src='A.js'>" + "</scri" + "ipt>"); parallelization only works in IE parallel downloads for scripts, nothing else all document.writes must be in same script block http://stevesouders.com/cuzillion/?ex=10014

22. Browser Busy Indicators

23. Browser Busy Indicators good to show busy indicators when the user needs feedback bad when downloading in the background

24. Ensure/Avoid Ordered Execution • Ensure scripts execute in order: • necessary when scripts have dependencies • IE: http://stevesouders.com/cuzillion/?ex=10017 • FF: http://stevesouders.com/cuzillion/?ex=10018 • Avoid scripts executing in order: • faster – first script back is executed immediately • http://stevesouders.com/cuzillion/?ex=10019

25. Summary of Traits *Only other document.write scripts are downloaded in parallel (in the same script block).

26. and the winner is...

27. Load Scripts without Blocking don't let scripts block other downloads you can still control execution order, busy indicators, and onload event What about inline scripts?

28. Inline Scripts Block long executing inline scripts block rendering and downloads http://stevesouders.com/cuzillion/?ex=10035 workarounds: initiate execution with setTimeout (>250 for FF, nglayout.initialpaint.delay) move JavaScript to external script with advanced downloading techniques use Defer attribute (IE only)

29. Inline Scripts after Stylesheets Block Downloading Firefox blocks parallel downloads when downloading stylesheets IE doesn't... ...unless the stylesheet is followed by an inline script http://stevesouders.com/cuzillion/?ex=10021 best to move inline scripts above stylesheets or below other resources use Link, not @import

30. Examples of Scattered Scripts MSN Wikipedia eBay MySpace

31. Don't Scatter Inline Scripts remember inline scripts carry a cost avoid long-executing inline scripts don't put inline scripts between stylesheets and other resources

32. Takeaways focus on the frontend run YSlow: http://developer.yahoo.com/yslow this year's focus: JavaScript Split the Initial Payload Load Scripts without Blocking Don't Scatter Inline Scripts speed matters you CAN make your site even faster!

33. Steve Souders souders@google.com http://stevesouders.com/