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It’s worth noting that on Flickr we’ve disabled this type of image deferral for IE6, 7 and 8.
We found that the cost of finding all of the nodes that need images inserted, and the impact on the page that downloading all of those images had, vastly outweighed the benefits we got by making those images load after the rest of the page has finished. It made the pages very sluggish and jerky, and significantly delayed event handlers from being invoked.
More modern browsers don’t have this problem at all, and are able to run the JS and download the images without any appreciable CPU load. We are still using this technique, with data-defer-src, for them. IE, though, gets img tags with normal src attributes instead.