External scripts referenced in the head of your pages are especially harmful because they delay the display of your body content. Delays before your body content displays make it more likely that users will bail out. Human-computer interaction (HCI) research has shown that delays before viewing pages are less frustrating than delays after a page has loaded.1
Post-loading delays are a common problem with Ajax-enabled pages. When it is poorly written, Ajax can make things especially difficult on narrowband users. Even with HTTP compression, the latency due to grabbing all those separate files can cause indeterminate delays. Ajax also introduces polling with the
The perils of third-party widgets
div, create a script element, and then append the
scriptelement to the
divwe created earlier and loads an image:
document.getElementById('wedje_div_example').innerHTML+='<img src="http://www. example.com/images/example.gif" width="60" height="60" />';
Another option is to use iframes to load ads, but iframes can ruin the context-sensing abilities of contextual ads and so you must use them carefully. For more details on WEDJE, see Mike Davidson's blog post on the subject. (Note that this technique has some strange Internet Explorer 6-related issues, but you can filter with conditional comments to use on only Internet Explorer 7 and later.)
 Dellaert, B., and B. Kahn. 1999. "How Tolerable is Delay? Consumers' Evaluations of Internet Web Sites after Waiting." Journal of Interactive Marketing 13 (1): 41–54.
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