@lukasbestle As you mentioned, the first load took “3 seconds”, the following page load around “3 milliseconds”. When you wait a while (a longer period), the next page load again lasts “3 seconds” - then the next page load “3 milliseconds” - and so on. (The “seconds” depends on the host, which is running the scripts)
After a hint in another post (Super slow frontend and panel not loading at all), I changed von PHP 7 back to 5.6 and activated fast-cgi.
This helped in that way, that the (first) load of the pictures lasts only a tenth of the time required before. But the first load of one (!) picture (of a dozen on this page) requires still approx. 500 milliseconds.