Basic rule of web designs - quick to download

Your web site should be quick to download

Studies have indicated that visitors will quickly lose interest in your web site if the majority of a page does not download within 15 seconds. (Artists' pages should have a warning at the top of their pages.) Even web sites that are marketed to high-end users need to consider download times. Sometimes, getting to web site such as Microsoft or Sun Microsystems is so difficult and time consuming that visitors will often try to access the sites during non-working hours from their homes. If your business does not have good brand name recognition, it is best to keep your download time as short as possible.

A good application of this rule is adding animation to your site. Sure, animation looks "cool" and does initially catch your eye, but animation graphics tend to be large files. Test the download time of your pages first. If the download time of your page is relatively short and the addition of animation does not unreasonably increase the download time of your page, then and ONLY then should animation be a consideration.

source: http://www.grantasticdesigns.com/5rules.html