Update: Deferredto is no longer available. A comparable extension that offers similar functionality is not available as well. End
Especially image heavy websites are often making use of a technique that is called lazy loading to speed up the time it takes to display the contents of a page on the screen.
The idea here is to load only those elements first that are needed to fill the visible part of the screen. Imagine opening a page with 100 images that are displayed in a column on the page. Instead of loading all or many of the images at the same time, only the images that you see when you open the page are loaded first, with the remaining images either being loaded when you scroll the page or after the loading of the visible content has ended.
Deferredto for Firefox uses a similar approach, only that it moves it from being implemented on individual websites - which you have no control over whatsoever - to the web browser so that it theoretically works on all pages you visit in the browser.
This works best for slower connections while high speed broadband users may not see a difference at all in their page loading times. The developer of the extension uses a placeholder system for images that have not been loaded yet using their width and height information so that the design of the website is not altered in any way by the extension.
If you are noticing slow downs, freezes or other issues while browsing image-heavy websites, you may want to check out the extension for the Firefox web browser to see if it improves your loading time of those pages in the browser. I'm on a 50 Mbit line and could not really notice any difference in regards to page loading speed (near instant is hard to improve).
The extension is based on the Deferredto script that the developer is making available on the official project website. Webmasters can use it to apply the same technology to their websites. Note that it requires you to load a third party script on your website.
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