please show your understanding of the data rate required by an uncompressed 1080p video.
yes, you’re likely causing VideoWriter to write an uncompressed video stream to file.
you should just use ffmpeg, and request MJPEG from the webcam itself (C920 do MJPEG) and store that. this doesn’t require OpenCV, which is for computer vision, not for moving video data.
you won’t be getting 1080p uncompressed from a USB 2.0 webcam. maybe at 5 fps, but not more.