Performance drops

After about 3-4 hours of flight, the frames rate drops from an initial 35 to about 15 frames.
What’s going on?

Mod edit (original text):
8. August 2021[date=2021-08-08 timezone=“Europe/Zurich”]

Nach ca. 3-4 Stunden Flug bricht die Frames Rate von anfänglich 35 Blidern auf ca. 15 Frames ein.
Was ist da los?

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This topic has been discussed before:

I’ve closed this thread as there are existing topics about the same issue, so that we can keep all responses to this issue together and importantly so votes don’t get split between them. Feel free to continue the discussion over on those threads.