I've tried in the advanced options of the "Js Slideshow" setting the width to 100 (for the laptop only) but it doesn't work.
Hi. There is no this settings in the Slideshow, because it does not depend on the Slideshow. Go to the settings of the section in which the Slideshow is located and switch to Fluid Row.
slideshow I don't understand whether with an addon, this one or another with only 1 image, it's possible to have a background image with the header (logo, menus, etc.) on top, like a slideshow. cf https://ascension.fr/ or https://www.altiplano-voyage.com/
Again, it does not depend on the Slideshow, but depends on your template. You must write CSS to make your header (Astroid header) transparent and place it in an absolute position. Then you will get what is in the your examples.
I don't understand about the preloader. It's only for the home page? and therefore with SPPB? Is it because I didn't set the SPPB parameter for production? Could this be due to the theme? Yet I haven't activated any optimization settings, neither for caching nor gzip for joomla?
Not related to SPPB. The preloader is turned on for the entire site and it will very much irritate users. I suppose it can be disabled in the settings of your template (Astroid settings)