Im my project there exist several very large gallery-sets with up to 7.000 pictures, that make each browser naturally very slow when loading all 7.000 Img-elements in a floatbox slideshow at once...

I try to incrementally split these sets e.g. in 100 pics per slideshow, but would like to load automatically the next 100 pics on the end of each previous set.

Is there a possibility to tell floatbox to procede with another set of pictures at the last picture of a gallery/slideshow-set?

Or what in general is the best approach to display such large sets of pictures in one consistant fb-slideshow?

If you're using a Floatbox version prior to 5.4.0, set preloadAll to false. This will prevent Floatbox from trying to fetch every referenced image at page load time. v5.4 and later use the preloadLimit option which defaults to 5 images for desktops and 1 image for touch devices. These defaults should be fine and don't need changing.

You are now optimal for a large slideshow and don't need to adjust anything further. When Floatbox shows an image from a gallery set, as soon as the image is up in the floatbox it then quietly does a pre-fetch of the next image so it's ready to go when it's time comes. For example, as soon as image 48 of 7000 is shown, image 49 is fetched into the browser cache. This approach results in a fairly quiet network but a responsive and usually seamless gallery presentation.