In the case of the street signs, the files are fetched from WM Commons once, because the wiki uses it as one file repository (among its own one). This feature is called “InstantCommons” and it is documented at https://www.mediawiki.org/wiki/InstantCommons. According to the docs, once, but the HTML of the wiki page rather looks like it needs to access WM Commons’ rendered PNG files at every single request.
It seems that the base html takes 42 seconds to load. The images are loaded fast.
But, when I reload the page (even with client-side caching off), Chrome seems to load the page fast. Is it possible that the server has to construct the page the first from different HTML-blocks and that this construction is slow?