so I see tons of people have exact same issue as me with c920 low fps etc, I have followed some of the other advice in the channel and support channel to no avail. My cam is connected direct to rpi4(real rpi4) no powered hub yet but I tried setting up with camera-streamer mode and using /webcam/webrtc results in zero picture. I only get picture if i add it in adaptive mjpg mode. The other day I was getting steady 15-30fps on this camera with my settings everything was BEAUTIFUL . I changed literally nothing and today it was back to low fps or not showing up at all.
I have also Made the camera-streamer folder clean as indicated in another post.