Thanks for your reply.
I play at 4K (my native Screen resolution) in this game.
Lowering the resolution still gives me an instant 9 fps drop as soon as I start recording.
Also I have many other games I recorded even at higher resolution (through DSR) without big issues.
This means it's not a general problem with every game but only with some games.
The steps suggested in the links you sent I tried but they didn't help unfortunately.
Thanks for looking into this issue.