Hi @andreas.markl,
Have you had a look at this FAQ: Optimizing Prismic API Usage: Why It Might Be Higher Than Expected? It might put you on the right path. Another one that may be useful to you is this one: Prismic API Limits and CDN Bandwidth explained: FAQ Guide - #2, if only to give you more information on what's happening here 
Let me know if you need anything more specific.