Lack of interest Implement asynchronous image decoding via <img decoding="async">

[Steffen]

To reduce rendering jank, asynchronous image decoding with <img decoding="async"> (HTML5 specification) should be used at least for attachment images which can be quite large (and maybe for avatars in the forum and thread views, too). AMP uses it for all images. It is supported in Chrome 65+, Firefox 63+ and Safari 11.1+. Microsoft Edge just ignores the attribute.

Ideally the gallery would use it, too, but I guess that's rather unlikely.