Thanks for your answer. I cannot show any more details because of organisational secrets. Hower, the find condition is pretty simple, nothing special.
If a missing index is the problem I’m wondering why the count query is so fast and the other so slow. In my understanding, an index helps in the engine to filter out the documents you want to do something with. If the filtering part were to be the problem, the count query would be equally slow and there would be almost no difference between them. But that’s not the case. So, what am I overlooking?