Ffmpeg is quite good for this

Depending on your hardware strength and what media you’re converting, the handbrake suggestion might be best. But I’m coming in to say that does take a lot of compute power. You mentioned watching cams, so if it’s popular TV/movie data you’re compressing, it’s often quicker to just download a different, lower resolution source from torrent.

Tdarr

let it watch a folder and it will convert what ever goes in that folder and shove it into a new folder.