Hi, thanks for your support. I am already using Twenty Seventeen theme.
I also did what you suggested, de-activating all plugins besides Simple Lightbox, with no more luck.
Then, I thought about looking for another website using Simple Lightbox, then simply went to your website, and realized it didn’t feature what I was looking for either.
Maybe I didn’t explained it properly, so let me reformulate : I expect an image displayed in a Lightbox to be integrated with these srcset/size attributes, so that it loads an optimized image based on the screen size, instead of the heavy original one to be reduced by the browser. It’s a waste of bandwith.
In my opinion, loading the full size image kind of makes sense with the initial WordPress behaviour (<=> on a dedicated page) because then it offers the ability to display it fullsize, whatever the screen is. In a lightbox, that fullsize function is not available, so there is no further hope to exploit such a big image (besides may be downloading it).