posted by Steve Souders, March 17, 2008 2:47 PMIE8 is the first browser to support downloading scripts in parallel. This example, with 4 external scripts, 1 external stylesheet, 1 iframe, and 1 image, loads in 10 seconds in Firefox and IE7 and earlier, but loads in only 4 seconds in IE8. The HTTP profile for IE8 is shown in the figure below. Two other important things to note. First, the components are not downloaded in the same order they appear in the page. The page order is:
script, stylesheet, script, iframe, script, image, script
but the download order is:
script, stylesheet, script, script, script, iframe, image
The reason for this is likely the second item of note, namely, IE8 supports scripts being downloaded in parallel with other scripts and stylesheets, but iframes and images block parallel downloading of scripts. As shown in the figure below, IE8 moved the iframe and image to the end of the download order and doesn't start downloading them until all the scripts are done. It's great that scripts can load in parallel. Even better would be for scripts to load in parallel with all other items, even if execution has to be split up.