A web page will not render and appear to the end-user as complete until all downloads requested in the page have been delivered.
If requests are made for elements that are not needed for the initial page-view, then this will introduce unnecessary delays.
Image tags that don't include the images dimensions delay the browser while it calculates the image's size and the placement
of any surrounding elements.
The HTML and Net panels in the Firebug extension for Firefox, or Fiddler will show the content and timing of the page as delivered to the client.
Make the dimensions of any images explicit.
Avoid Redirects
Remove duplicate scripts
Post-loading, pre-loading
and lazy-loading