Anyways so it can be the game which causes the problem using cpu for streaming right? So warzone 2 ist just bad optimization. (Coded) ?
Pretty much all games are highly optimized. It's fascinating to see how early gaming systems were pushed to the limits of what they could do, and how the programmers of that time did it. Especially what tricks they used, when the hardware didn't officially support what they made it do anyway.
Modern games are the same way. The numbers are higher now, but there's still no way that a simple, straightforward program could do what a good game does, in fact, do. (or at least *appear* to do, which is all that matters) Gaming is still very much an exercise in squeezing every last bit of performance out of a system, even if the code has to be convoluted to get it. Bad optimization just doesn't compete in that environment.
So there's that, plus the massive load that you're adding by trying to encode high-spec video on the same general-purpose CPU, all in real time. Like I said, nothing is infinite.
If there's a section of GPU that is dedicated to video encoding, and the game can't use it for anything else (or at least *isn't* using it), then it's effectively free processing power to use it for its designed job.