is it possible to display a videostream with opencv which has been encoded in h264?
Yes, you should be able to read it with VideoCapture from a file or an RTSP stream with the FFmpeg backend.
why do you ask? what did you try? what precisely happened when you tried?
I have tried to display them with imshow() and this resulted in wierd gray lines through the frames. I dont know if it is possible to do it without alot of compute power as im having 12 video streams in real time coming into my pc with all need to be decoded and displayed. My threadripper 32Core cpu can just handle it but thats not an ideal setup. If there would be a less compute intensive way to do so that would be amazing 
To clarify you want to decode 12 video streams on a lower end cpu than a threadripper or you want to put less stress on the threadripper? What resolution are they?
I am unable to decode more than say 6 1080p streams on a low end 4 core Intel CPU. You could try using hardware decoding if you have an intel CPU or nvidia GPU.
threadrippers are not low end, so if that thing has trouble, the video streams must be extra high resolution and high frame rate.
you need to precisely tell us the properties of those streams. numbers. facts.
same with the “weird gray lines”. spare the words if a picture says it better.