IE8 speeds things up

March 10, 2008 9:30 AM

IE8 Beta 1 has several performance improvements listed in the release notes. Many of these improvements center around the DOM and JavaScript execution. And their announcement about stricter standards compliance is a great move forward. There are three changes that are big and relate to my performance best practices: 6 downloads per hostname, loading scripts in parallel (the biggest improvement!), and support for data: URIs.

Increasing parallel downloads makes pages load faster. (For users with slower CPUs or Internet connections it could possibly be worse, but for most users it’s faster.) The HTTP 1.1 spec recommends that browsers only download two items in parallel per hostname, but the spec was written in 1999. Today’s clients and servers can support more parallel downloads, so IE8 has increased the number of downloads per hostname from 2 to 6.

Increasing parallel downloads makes pages load faster, which is why downloading external scripts (.js files) is so painful. Firefox and IE7 and earlier won’t start any parallel downloads while downloading an external script. These days, with the greater adoption of Web 2.0 and DHTML, many sites contain multiple scripts which means those pages will have long periods where all other downloads are blocked. It’s understandable that these scripts need to executed sequentially (code dependencies) but there’s no reason they couldn’t be downloaded in parallel. And that’s exactly what IE8 has done. It’s the first browser I’ve seen that has implemented this critical improvement for load times. Facebook has got to be thankful for this. They have 17 external scripts on their page. In most browsers this causes the page to load slowly for users coming in with an empty cache. But for users coming in using IE8 the scripts load ~80% faster because they’re loaded in parallel. In this screenshot showing HTTP requests for Facebook we see parallel script loading, and we also see them loading 6 at-a-time. (Scripts are the icons that look like a scroll.) Both of these IE8 enhancements dramatically speed up pages.

The long awaited support for data: URIs in IE has arrived. data: URIs allow developers to embed resources (images, scripts, anything referenced via a URL) directly in the HTML document as described in the examples from the IE8 release notes. This is especially useful for CSS background images because you can embed the image’s data in the stylesheet which can then be cached.

Hats off to the IE team. I’m really psyched about their focus on performance and can’t wait for wide adoption of IE8.

-Steve

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