unless I am missing something the lazy loading option can't be changed at different breakpoints, its either on or off
so the whole concept of "lazy loading" is that the browser doesn't actually load the image if it is below the fold (out of view for want of a better word), until that image comes just into view, or just below where it comes into view, therefore helping with the page loading speed, and or if the user never gets to those images below, then also saving bandwidths usage.
so on mobile device breakpoints if there are images "Below" the fold then they require lazy loading enabled, as one moves up, some of those images move up and are then no longer below the fold, therefore no longer need "Lazy Loading" enabled but if you then un-select the "enable lazy loading tick box, it applies is across all breakpoint, in other words on mobile device widths it also becomes disabled (which sort of defeats the object).
It's only a small thing and perhaps some would say what's wrong with having lazy loading enabled on images at all breakpoints, the answer is that it isn't considered to be good practice, and it sometimes can result in not a very good UX for visitors, especially if at larger device widths the image is quite large (resulting in an image loading progressively,
The whole idea of lazy loading is super and has been eagerly awaited to be available for us within Site Designer (except for writing ones own custom code to apply it which has been an option for some time)
and the last point, in my humble opinion. the option to enable "Lazy loading" should be the user's option to enable it, not as a default (due to the reason, that if it doesn't require lazy loading, then it hasn't got it, if the user decides that it needs lazy loading, then it is their choice not the other way around.
Just some thoughts
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