Loading is super slow and I dont know if it’s my fault or there is some Kirby limitations (number of subpages or thumb gererations - maybe size of original images).
Hm, the thumbs are only created once, so that can slow down your site on first loading, which is usually done by yourself. So even if the original images are big, that should not affect the final page.
Showing 70+ subpages, however, can slow your page down, because all the html needs to be generated and then the images need to be loaded.
Maybe you can use caching and/or pagination to improve load times.