// Look at the slide below. How does your website compare with those numbers? A slow website means higher bounce rates (people come but wont load another page irritated or disinterested by the lack of speed), lost revenue and generally a low user satisfaction.
For a "crash course on web #performance" check out this presentation by Google's http://bit.ly/VJLjgt
If you have more time, there is also a 1 hour speech by Ilya at HTML5DevConf here http://bit.ly/Zw2szS