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@Jake – Styles in the <head>, as well as scripts that precede the images (and are not defered) must be downloaded, parsed and be applied/run before the img’s <script> can run and initiate the image’s download. Of course, it’s best to avoid using them, but it’s not always possible.
If the above resources are either relatively big, numerous, or delayed by network conditions (i.e packet loss, high latency combined with multiple resources, etc), this can stall the download of the images, possibly offsetting the time we “gained” by downloading the small resolution images.
I’m not sure what the performance impact of the “inline script per image” approach, but it may also add some delays to this process.